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I'm overhauling the brakes on my SA and I've run into a soft pedal issue I can't solve. I replaced the rubber brake lines, master cylinder and RL wheel cylinder (RR was replaced in Dec. '18) with parts from Centric since OEM are NLA. I also removed, cleaned and painted the brake booster and cleaned the proportioning valve. The master cylinder was bench bled - first by itself, then again with the proportioning valve. I adjusted the brake pedal free play and booster piston to master cylinder clearance to FSM specifications. I adjusted the rear shoes so I could just turn the drums by hand. I bled the brakes the usual way (pump, pump, pump, hold) starting with the RR, then the FR, FL and finally at the master cylinder again. By all means everything was completed by the book. The pedal feels good when the engine off. There's 1/2-1" of travel. The problem is when the engine is started. The pedal immediately drops more than 1" with zero effort and then becomes solid. Pumping makes no difference and once the pedal stops it maintains position (doesn't drop slowly). A short test drive showed very little braking power but if I really stomped on the pedal I could get something (the rear?) to lockup briefly. To fix this I've:
-Confirmed no leaks at any fittings and no leaks or bulges in the rubber line.
-Tried another master cylinder (brand unknown) billed as OEM from a well-known rotary shop.
-Disassembled, cleaned, reassembled and reinstalled the original master cylinder.
-Disassembled, cleaned, reassembled and reinstalled the proportioning valve.
-Locked the rear shoes against the drums prior to bleeding.
-Bled the brakes countless more times and tried using other methods: Motive pressure bleeder at >15 psi and the older method of crack bleeder, push pedal to floor, allow pedal to come back up, then close bleeder.
-Vacuum tested the brake booster up to 20 in. Hg. Confirmed pedal gets soft when vacuum is applied and hard again when vacuum is released.
-Blocked all ports at the master cylinder and confirmed the pedal was still firm. Pedal movement dropped to <1/2" but applying vacuum still results in no resistance over that 1/2".
Is there some trick or nuance to the SA brakes that I'm not aware of? I've completely rebuilt the brake system on my FC and replaced master cylinders, calipers, etc. on all my other cars and I've never had a problem like this.
Does it go all the way down? My SA pedal goes down 60% before it truly starts to stop the car, but that 60% gives the brakes a ton of nuance not found in modern cars. For reference my 73 Mach 1 mustang feels almost identical, don't expect to get solid stoping power like a new car where a half-inch locks the freaking wheels up. . I bled the brakes and installed new lines and sourced a NEW master cylinder back in 2018 , not OEM but they look nearly identical.
I will be honest, I rebuilt my brake lines the hillbilly way, just listened to what others told me here, bled them my self with a plank and a brick .. all I can say is my car stops, I do plan on having shop bleed them, but if you literally have no stoping power SOMETHING is broken. Redid all the brakes myself self etc.. Sounds to me like that booster is busted somehow, if everything was done right and your lines are not bleeding and the master is solid, something tells me the booster isn't functioning properly even if you are getting good readings from it
Does it go all the way down? My SA pedal goes down 60% before it truly starts to stop the car, but that 60% gives the brakes a ton of nuance not found in modern cars. For reference my 73 Mach 1 mustang feels almost identical, don't expect to get solid stoping power like a new car where a half-inch locks the freaking wheels up. . I bled the brakes and installed new lines and sourced a NEW master cylinder back in 2018 , not OEM but they look nearly identical.
I will be honest, I rebuilt my brake lines the hillbilly way, just listened to what others told me here, bled them my self with a plank and a brick .. all I can say is my car stops, I do plan on having shop bleed them, but if you literally have no stoping power SOMETHING is broken. Redid all the brakes myself self etc.. Sounds to me like that booster is busted somehow, if everything was done right and your lines are not bleeding and the master is solid, something tells me the booster isn't functioning properly even if you are getting good readings from it
The pedal doesn't go all the way down. With the engine off I can move it 1-1.5" or so with increasing resistance before it stops. It feels normal and has a decent feel for an old car. It's similar to my FC. The issue is when I start the engine and vacuum is applied. That 1-1.5" now has zero resistance and I get no braking force. It feels like the MC isn't even there. It doesn't make a difference if I slowly depress the pedal or smash it. Once the pedal hits resistance I get some breaking force and if I really lean into it quickly I can lock something up.
The brakes worked fine before this. I've owned the car for 4 years now so I'm familiar with how the brakes should feel. The only reason I did anything is because I've been driving around with 42 year-old rubber lines, which seems like a bad idea. My records show the MC was replaced in '88 so I figured it was due as well. The first time I tried this I literally just replaced the three rubber lines and MC then bled the system. It was when that didn't work that I started checking and adjusting the rear shoes, pedal free play and booster pushrod to MC clearance. That made a massive improvement in pedal feel with the car off but didn't fix the issue. Then I tried another new MC. When that didn't work I tried the original MC again. Unless both new MCs I received are bad, and I damaged the original in the process of disassembling, cleaning and reassembling it, it doesn't seem like this is a MC issue.
The only other parts I really touched are the booster and prop valve. I ultimately ended up taking the prop valve apart and cleaning it. Nothing looked out of the ordinary so I put it back together. That didn't do anything. I am suspicious of the booster but it doesn't make sense. Experience tells me a failed booster or no vacuum results in a hard pedal and no assist. There's some anecdotal evidence online of people fixing low/soft pedals with a new booster but never any explanation how that addressed the problem. I literally just cleaned and painted it anyway. It was never dropped or banged into anything. It was never submerged in water or truly even soaked. Just sprayed down with degreaser. I was able to source another used booster and prop valve so I'm going to try them when they arrive.
The only other thought I've had is the front calipers. While I did not remove them the old lines were a bear to get off and I definitely twisted/moved the calipers a bit. I don't see how that would have done anything though. There's still a decent amount of drag on the rotors so it's not like the piston was pushed back into the bore. Even if that was the case, the piston should have extended back out during the bleeding process.
Originally Posted by j9fd3s
the pedal on these cars is not setup to modern tastes, it has more travel than a new car.
that being said, the rear brake shoes and the adjustment are super critical, i'd recommend new rear shoes and adjust them to spec that helps a lot.
and you might check the clearance between the master cylinder and the booster, i've seen that be off too
The car drove and stopped fine before this. It only has 27xxx miles on it so there's lots of pad and shoe material thickness remaining. The pedal free play, booster pushrod to MC clearance and rear drums have all been adjusted per the FSM. Free play is around 8 mm, which is within the 7-9 mm range. Booster pushrod clearance might still be slightly out of the 0.1-0.5 mm spec but it's much better than it was before. Assuming I was measuring everything correctly the original clearance was around 5 mm. I find this strange because all 3 MCs have the same bore depth. So either my car wasn't adjusted correctly at assembly or someone played with it in the past. The rear shoes are adjusted such that attempting to spin the drums results in 1/4-1/2 rotation at most. I'll admit doing all this has greatly improved pedal feel and travel with the engine off. It really feels little different from my FC. It's just when vacuum applied that I lose pedal resistance and braking power.
It's your booster. The pedal feel when shut down is irrelevant, The booster is not functioning when your car is shut down, literally, nothing changes but that when you turn your car on.
It's your booster. The pedal feel when shut down is irrelevant, The booster is not functioning when your car is shut down, literally, nothing changes but that when you turn your car on.
I'm really hoping it is. Won't know for sure until I try another one. It's just weird because my understanding is a failed booster just results in a hard pedal with no brake assist. This feels like it's either over-boosting the brakes or the pushrod isn't moving with the brake pedal but I don't understand how either of those are possible. The other thing is it was functioning perfectly fine before and all I did was remove, clean and paint it. Maybe if I had dropped it or completely submerged it in cleaning fluid but neither of those happened.
I've run across a few clutch masters not tightening up to the firewall due to the new studs not being threaded far enough, but not brake masters.
I agree, just removing the B/M to clean and repaint it should not affect it. Your vacuum line is intact and not leaking is it? It has a one way valve in it that should have the arrow pointing towards the firewall. It isn't backwards is it?
I'd be curious, if you jack the car up and apply the brakes while not running, does it abruptly lock the front and / or rear wheels? What about while running?
A lot of the newer MC have a design flaw where the bolts are not threaded all the way and wont tighten up completely. Just a guess.
With the brake MC the studs are on the booster. When I cleaned up the booster I chased the threads to make sure they were good all the way down so everything is tight when bolted together. It doesn't look like the MC is moving.
Originally Posted by Banzai
I've run across a few clutch masters not tightening up to the firewall due to the new studs not being threaded far enough, but not brake masters.
I agree, just removing the B/M to clean and repaint it should not affect it. Your vacuum line is intact and not leaking is it? It has a one way valve in it that should have the arrow pointing towards the firewall. It isn't backwards is it?
I'd be curious, if you jack the car up and apply the brakes while not running, does it abruptly lock the front and / or rear wheels? What about while running?
Despite being old, the vacuum line and check valve are in place and working. Unless the check valve somehow regulates the amount of vacuum the booster sees I don't think it's possible it's causing the problems I'm having.
The car has been in Back to the Future: Part II mode for a few weeks now. With the engine off there's a progression of braking force. It feels similar to my FC. With the engine running that progression is gone. Pedal resistance goes from nothing, nothing, nothing then hard where it should get progressively harder to push. I've been playing with connecting and blocking various circuits in the braking system. With all the ports on the MC blocked there is very little pedal movement. It's whatever the pedal travel is for that <1/8" piston movement in the MC. As I connect the brake circuits the pedal travel increases. The biggest increase in travel seems to come when the front brakes are connected. For some reason it's taking a good amount of pedal travel for the discs to lock up, which doesn't make sense. With no force on the pedal there is drag on the discs. I even cleaned up and lubricated the wedges and contact surfaces to make sure the calipers weren't binding. It feels like either there's still air in the lines or the fluid is going somewhere else first before applying pressure on the pistons in the calipers. Originally, the calipers were only touched when I replaced the rubber brake lines. It's only since this problem began that I pulled the calipers from their brackets and started to clean/lubricate things. There are no signs of leaks around the dust boots.
Some of these things were mentioned before so if you have covered these things sorry for the duplication.
With the car running and vacuum to the booster can you push the pedal quickly and hard without the increased travel? In other words is the increased travel worse when lightly applying the brakes? If the booster is going bad and air / vacuum can bleed by the diaphragm and the pedal will not stop and be soft until full travel is reached and the booster isn't adding any force.
I am leaning towards the booster because this much travel, air in or fluid out should be fairly apparent. In particular, are you getting air out of the calipers when you re-bleed them, although it doesn't take a lot of air to effect the feel of the pedal, that much travel I would assume you would get air out of the brakes when you bleed them. The same goes for fluid, is the fluid level in the mater cylinder going down? Again enough of a fluid leak to cause the pedal to travel that far should be apparent, think how much fluid you get while bleeding the brakes with one push of the pedal.
Some of these things were mentioned before so if you have covered these things sorry for the duplication.
With the car running and vacuum to the booster can you push the pedal quickly and hard without the increased travel? In other words is the increased travel worse when lightly applying the brakes? If the booster is going bad and air / vacuum can bleed by the diaphragm and the pedal will not stop and be soft until full travel is reached and the booster isn't adding any force.
I am leaning towards the booster because this much travel, air in or fluid out should be fairly apparent. In particular, are you getting air out of the calipers when you re-bleed them, although it doesn't take a lot of air to effect the feel of the pedal, that much travel I would assume you would get air out of the brakes when you bleed them. The same goes for fluid, is the fluid level in the mater cylinder going down? Again enough of a fluid leak to cause the pedal to travel that far should be apparent, think how much fluid you get while bleeding the brakes with one push of the pedal.
Hopefully this helps.
Mat
No, pedal is the same whether it's depressed slowly or quickly. Pumping doesn't do anything either. The booster holds vacuum indefinitely. It just seems like it applies all assist immediately. I was under the impression the assist should increase with pedal travel. I'll admit I only have a general understanding of how brake boosters work (vacuum acting on a diaphragm and all that). I don't know if there's some kind of valve that's supposed to regulate how much vacuum is lost/maintained on the backside.
The only air I get out of the bleeder is the air that gets sucked in past the bleeder threads (only on the RR). For the fronts it's a steady stream of fluid. If this was air it'd have to mean a pretty large pocket it stuck somewhere. I definitely know I don't have a leak anywhere. I've put more pressure on the brake pedal than I would in a panic stop and checked all the fittings. I've swapped the MC in and out many times and there's never any signs of fluid in the booster.
Originally Posted by KansasCityREPU
Crazy question, are the calipers on the correct sides with the bleeder pointing up?
These are the older wedge-style calipers and the bleeders are near the top pointing toward the rear of the car. I never really removed the calipers from the car. The first time they were touched I just replaced the rubber lines. The calipers stayed in place. The second time I removed the wedges to clean and lubricate everything but the rubber lines remained attached.
Unfortunately, the potential replacement booster and proportioning valve are garbage. Working with the seller on that...
I've been scouring the internet for people with similar problems - many on these forums. More often than not the issue is solved with a new master cylinder. While it seems unlikely I have 3 bad MCs I'm throwing caution (money) into the wind and going to try another one. The problem is, there are no '79 specific MCs available that I can tell. So I went ahead and ordered one from Beck Arnley for an '80 from Rockauto (last 3 were stock, Centric and unknown). Does anyone know if there's a difference between the '79 and '80 MCs? OEM Mazda lists two difference PNs. So does Centric. Beck Arnley offers one for the '80 but not '79. However, both Atkins and Mazdatrix list '79/80 as being the same.
Does it seem at all remotely possible that a MC would check fine on the bench (pistons move <1/8"), feel fine with the engine off, but when vacuum assistance is provided there's now enough force to blow fluid by one of the pistons internally? Now have that happen three times in a row?
Unfortunately, the potential replacement booster and proportioning valve are garbage. Working with the seller on that...
I've been scouring the internet for people with similar problems - many on these forums. More often than not the issue is solved with a new master cylinder. While it seems unlikely I have 3 bad MCs I'm throwing caution (money) into the wind and going to try another one. The problem is, there are no '79 specific MCs available that I can tell. So I went ahead and ordered one from Beck Arnley for an '80 from Rockauto (last 3 were stock, Centric and unknown). Does anyone know if there's a difference between the '79 and '80 MCs? OEM Mazda lists two difference PNs. So does Centric. Beck Arnley offers one for the '80 but not '79. However, both Atkins and Mazdatrix list '79/80 as being the same.
Does it seem at all remotely possible that a MC would check fine on the bench (pistons move <1/8"), feel fine with the engine off, but when vacuum assistance is provided there's now enough force to blow fluid by one of the pistons internally? Now have that happen three times in a row?
You are spending a lot of money on parts you know work, get that booster swapped out, my MC from rock auto works just fine.
You are spending a lot of money on parts you know work, get that booster swapped out, my MC from rock auto works just fine.
Yep, and I pulled the trigger on that 4th MC too soon. I guess I'll have plenty for the years to come...
I might have found the solution. Took something like 15 pages of Google searching through this site but this looks like as good a solution as any: https://www.rx7club.com/2nd-generati...r-fix-1103221/. That thread links to an AusRotary thread that talks about a rubber "reaction disk" that sits underneath the booster pushrod. A quick Google search reveals this thing falling out is a common problem on 240/80Zs. There doesn't seem to be a good explanation as to what this thing actually does but based on the problem it solves for the 240/80Z guys it appears to have something to do with regulating the booster assistance.
I pulled the booster and, sure enough, that disk had fallen out. Remember how I said my booster pushrod to MC clearance was off by like 5 mm? Want to take a guess how thick the reaction disk is? I cleaned up the pushrod an seals, gave everything a good coat of grease and epoxied the disk to the bottom of the pushrod. Got the pushrod to MC clearance set again and called it quits for tonight. Tomorrow I'll get everything back together and test again. Will report back with results.
For anyone curious, the offending piece is the little rubber disk on the far right:
It sits underneath the booster pushrod inside a little cup in the booster. It's not firmly attached; just held in place with grease so it likely fell out when I was cleaning, sanding and painted the booster since I was moving it all around.
Glad you found the culprit. I knew I had to be the booster since you pedal felt fine when turned off. the only factor that changes when your car is on is the booster pressure. A faulty master cylinder usually leaks or sinks all the way down when depressing the pedal EVEN when the car is off. When my master cylinder died, I felt it immediately and could audibly hear the fluid spraying. When I got home my fluid was downright above low ( and I had only driven about a tenth of a mile from the incident back to my home) If this fix doesn't work I highly suggest you do source another MC. I can see the pushrod is corroded, that a symptom you HAD an MC leak at one point in time, that leak may have damaged your booster but its too early to tell until you test your fix out.
Also just for next time, when multiple parts don't work out of the box, its usually not the parts that are causing you trouble. I can understand individual parts being crappy and not being all that great, but parts being outright nonfunctional out of the box usually means something else is failing. I understand they sell MC repair kits as well, I still have my stock MC if you wish to have it, il give you a cheap price for it since it is "broken"
Got everything buttoned back up this afternoon and confirmed the brakes are working normally. With the new MC, lines and fluid and properly adjusted drums they feel better than ever! I still have no idea how this reaction disk functions as part of the brake booster but it definitely has something to do with controlling the amount of assistance it provides. So there we go. It IS possible for a brake booster to cause a low/soft brake pedal but it's incredibly uncommon. For anyone having brake problems, if you've never removed and messed with the brake booster it's unlikely this is your problem.
Originally Posted by Frogman
I highly suggest you do source another MC. I can see the pushrod is corroded, that a symptom you HAD an MC leak at one point in time, that leak may have damaged your booster but its too early to tell until you test your fix out.
Also just for next time, when multiple parts don't work out of the box, its usually not the parts that are causing you trouble. I can understand individual parts being crappy and not being all that great, but parts being outright nonfunctional out of the box usually means something else is failing. I understand they sell MC repair kits as well, I still have my stock MC if you wish to have it, il give you a cheap price for it since it is "broken"
Those parts are from a scrap booster in that picture. The one I'm using is in much better condition and only had some mild surface rust on the booster body from a leaking MC in the past but it doesn't look like anything actually got inside. That being said, I do now have 3 spare MCs: the original, which doesn't leak plus three "new" ones. I guess I have plenty to last me a lifetime now. I usually don't throw parts at problems but given the prevalence of bad rebuilt MCs, calipers, etc. I had to be sure. Fortunately they aren't that expensive.
This is one of those odd, quirky issues you run into from time to time. A good one to note and file away for future reference. Glad you got it figured out. So you have a couple MC's that were installed but unused? Might be interested in one if you wanted.
This is one of those odd, quirky issues you run into from time to time. A good one to note and file away for future reference. Glad you got it figured out. So you have a couple MC's that were installed but unused? Might be interested in one if you wanted.
Yes, I have 3 though I'll be keeping the "original" (replaced in '88) for reasons. So I have Centric 13045100 and Beck/Arnley 0728104. The Centric is listed as being for a '79 and the Beck/Arnley for an '80. Atkins and Mazdatrix show the same PN for both but the Mazda Parts Fiches show different PNs. I honestly don't know the difference, but if you want one of them let me know. I really have no use for 3.
NICE piece of detective work! Impressed by your determination (like you had a choice?). Thanks for posting the trials and the solution. Everybody wins!
09-30-20, 04:10 PM
