Hey all,

Just finished rebuilding my brake calipers, installed new disk brakes in the front, new pads and shoes, etc. Several months ago I noticed some brake fluid was leaking on the interior side of the firewall right down onto the driver side floor pan. I know for sure the clutch master is leaking the fluid (gonna replace it here soon), but couldn’t tell if the brake booster/master cylinder was leaking some of that fluid too, as it looks a little moist around the arm connecting to the brake pedal.

I then removed it thinking I would swap the one from my parts car which is in better shape, but that one is no good, and so I’m feeling like I’m out of options aside from buying a used/refurbished one. Again I have no idea if the brake booster is even leaking or not, I just felt like being extra cautious and throwing in something reliable before filling the master cylinder with fluid (I do know, however, that the brake master cylinder doesn’t leak at all).

Any thoughts? Even if it’s a little leaky should I just leave it? (Which I’m inclined to do).

I’ve attached a video of the side I am slightly worried about, which also shows how sloppy the arm that connects to the pedal is.

Thanks for looking!

 

Your options is to have the booster rebuilt or get the techno toy tuning small booster. I went with the t3 booster but it sucks. Doesn't stop like a stock booster. It's quite a bit smaller than the stock one.

 

If its not broke don't fix it! I just looked thru rockauto and no RX-7 brake booster are in stock at all for any model year. You could try sourcing it from Japan but that will cost $$$.

 

NB non abs miata booster has been really good for me. It was direct bolt on but you will need to fab a couple of hard lines to the prop valve.

 

Awesome, thanks so much guys! I think for now I’ll just reinstall it, see if it leaks, and if it does I’ll look into replacements

 

The SA share a lot of weird parts with the NA and NB Miatas. I'd never heard this before, good stuff.

@MTheoryInc The bolts for the brake master look wider on the miata booster. Are you running a stock master or something else?

 

I'm using the NB Miata master cylinder also.

 

Well, I’ve run into some more issues…. I can’t get the brake system to bleed whatsoever, and I’m also wondering if the brake MC is leaking fluid into the booster?

When I got the car the MC was bone dry and the brakes hardly worked. I’ve since rebuilt the calipers, new hoses in front, new wheel cylinders, etc., and I’m just now trying to bleed everything, but to no avail… The fluid is just simply staying in the MC reservoir, fluid level unchanging, as I press the brake pedal down, and only air comes out of the bleeder screws.

Another thing to keep in mind is when I pulled the brake booster off it was 1/3 of the way full with brown brake fluid…

I also noticed that the backside/interior side of the brake booster is sucking air in from around the brake pedal as I press it in, as shown in the video attached.

What do you guys think might be wrong here?

Thanks,
Pascal

 

Did you replace the master cylinder?
Depending how long it sat, being full of brake fluid isn't good for the rubber diaphragm inside the booster.

the booster works by having the engine create a vacuum within, then atmospheric pressure from outside the booster multiplies pedal pressure, so the sucking sound from around the pedal is normal.

Air being pushed out the bleeders means at least something is pushed from the MC. You could also try gravity bleeding: just leave the bleeders open with a hose on them and wait for fluid to come out. That way requires patience, just check on the reservoir a couple times an hour.

 

You are bleeding the rear first right? And it has a drum rear end I assume. It takes a lot of fluid to get from one side to the right rear where the bleeder is, especially since you put in new hoses. The whole length of tubing is full of air. Also the rear drum brakes have a brake bias module that sometimes gets stuck and doesn't let fluid pass thru. Usually that can be unstuck by pounding the brake pedal a few times really hard. I've had that happen before too on a drained system.

 

Might wanna check the integrity of all the connections and fittings. Make sure you don't have a small leak somewhere that is allowing the fluid to escape before reaching the bleed screws. Same goes for any holes or breaks in the hard lines.

 

Thanks for the great replies guys.

Surprisingly I was able to quickly bleed the rear axle first thing, but then went to the front calipers and MC and had no luck, just kept getting air.
I checked every brake line fitting, and each is nice and snug, no leaks there.
The car currently doesn’t run, so I assume that means there just isn’t enough pressure from the booster + MC to get the fluid flowing through everything? Which would be weird because the rear axle bled just fine… 🤷‍♂️

 

The front and rear are independent circuits. Could be a blockage somewhere. If you disconnect the 2 front lines where they connect to the proportioning valve and open front both bleeders you should be able to blow compressed air through both right and left lines. That would isolate it back to the proportioning valve / master cylinder, which I suspect it where the problem may be as the chances of both front lines being blocked is pretty slim.

I use a small vacuum bleeder that you attach to the bleed screw and draw the fluid through the line from the reservoir. You just need to keep an eye on the fluid level and not let it drain the entire reservoir before re-filling it. I've had good luck with speed-bleeders as well. One man operation, just pump the pedal, no opening and closing the bleed screws btw pedal pumps.

 

Okay great, thanks so much. I took the hard brake line off of the front of the MC to the proportioning valve (not the line running from MC directly to driver’s front caliper), gave the pedal some pumps, and zero brake fluid came out… I did the same for the line to the “rear” side of the prop. valve, and only a small amount of old fluid came out, none of the new stuff I poured in the MC reservoir… I’m thinking I need a new MC?

Also when removing that one-way valve with the arrow on it going from the brake booster to the intake manifold, I can hear a liquidy sound, and like I mentioned previously, I found lots of brown brake fluid in the booster, so I assume it’s leaking into the booster pretty good…?

What do you all recommend? Just a new MC?

Thanks so much,
Pascal

 

It should have been replaced when you found brake fluid inside the booster.

 

Haha okay 😅 thanks so much!

 

Since you're replacing the master cylinder and brake booster soon, it's probably worth reading this entire thread, specifically the linked post about the reaction disc

https://www.rx7club.com/1st-generati.../#post12437745

 

​​​​​​Rockauto hasn't had brake boosters for years. There's a place in San Jose CA. That u can send it in to be rebuilt. I have the t3 super small booster that I'm not to happy with. Now after reading this I'm going for a Miata booster. Does anyone know the diameter of the Miata booster?

 

I don't but it also has a different bolt pattern and you have to use a miata master cylinder as @MTheoryInc mentioned earlier in the thread.

 

Doesn't sound like adapting the Miata mc is that big of a deal. The booster looks like it would fit the sa without moving the clutch mc over. Big plus.

 

Miata booster is slightly wider but doesn't interfere with the clutch master.
The back needs a spacer so it clears the rib along the firewall, I made a drawing for one in this thread.
The bolt pattern for the brake master cylinder would now be compatible with a wide range across different makes and models.

 

I only mentioned it because if you have an SA the threads for the brakes lines are not compatible. For the FBs it surely is an easy swap to use the miata booster/MC.

 

I took j_tso drawing and make a step file for 3D print. The 3D print can be used as a test only.

 

Wish I knew about the Miata booster option before I bought the t3 booster. My SA has all fb lines and brakes. So should be a easy swap. Have a spacer from my fb booster too. Anyone want to buy a brand new t3 booster? Have a good fb booster too.

 

MTheoryInc, do you have an SA or FB with miata booster?

My 79 has a 7” booster and it looks like I have about 1/2” between the booster and the clutch master. Rockauto lists the Miata booster as 8.5”, so wouldn’t fit in my case. I remember reading that the 80 SA had more clearance between the booster and the clutch master, and the FB’s had even more clearance.