Loss in idle rpm when high brake pedal pressure is given , brake booster ? Help plz
07-12-07, 08:03 AM
#1
Loss in idle rpm when high brake pedal pressure is given , brake booster ? Help plz
I think my car has a problem with the brake booster or something with the brakes, because if I'm at a stop the idle will drop more and more if i press the pedal down with as much pressure as I can.
Not only do I have that problem but more importantly , sometimes when I am getting on the brakes while driving or just holding them down at at light the pedal itself will lose pressure and if I am driving while this happens I lose alot of braking power which is a really bad thing. I have all new brake pads and rotors and Im running a 5 lug setup (4 piston front calipers)
Any help would be greatly appreciated.
Also there are no visible leaks from the brake master cylinder, just thought id throw that info out there.
Not only do I have that problem but more importantly , sometimes when I am getting on the brakes while driving or just holding them down at at light the pedal itself will lose pressure and if I am driving while this happens I lose alot of braking power which is a really bad thing. I have all new brake pads and rotors and Im running a 5 lug setup (4 piston front calipers)
Any help would be greatly appreciated.
Also there are no visible leaks from the brake master cylinder, just thought id throw that info out there.
07-15-07, 01:18 AM
#3
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the big vacuum line that goes to the boost gets very brittle being over the exhaust manifold. it will crack on the bends so you wont even notice a leak until you apply pressure.
07-15-07, 02:00 AM
#4
When you say the pedal itself will lose pressure while driving, do you mean to say that the pedal travels farther than normal before the car slows down or that the effort required is higher than normal? it would help more to know this. If the brake pedal goes down too far before braking occurs, and the brake fluid level hasn't dropped, most likely it's a failing master cylinder. If the pedal requires more effort than normal (harder pedal) but the pedal's height is normal, then suspect the booster or vacuum supply to it. When you remove the vacuum hose to the booster with the engine idling, there should be a strong suction felt with your thumb on the end of the hose. If not, check the hose like Walken said.
07-15-07, 02:19 AM
#5
Another check for the booster is to pinch the vacuum hose with a pair of pliers while the engine is idling, cutting off vacuum to the booster. If the idle speed drops when the hose is closed off, there is a leak in the booster's diaphragm. The hose may be brittle and crack when you pinch it closed, so be prepared to replace it if it does.
07-15-07, 07:56 AM
#6
Presumably the Mazda replacement line incorporates the valve.
07-15-07, 09:38 AM
#7
Where can i purchase a new one ? I think this may very well be the problem, it at least sounds like a good starting point to fix it.
I really think it is the hose because when ever I am applying pressure the brake pedal will feel ok at initial braking, but the pedal will eventually travel to the floor as far down as the pedal can go and braking will become weak. Even if I am holding the brakes down at a stop light this will happen.
Im most certain its the hose because I remember seeing a crack in it , but it was only visible when you bend the hose a certain way...but Im sure when given pressure the crack will expand because it cant hold the pressure.
So yeah where can i get the hose ? I kind of need that so I dont crash my car, luckily the crack hasnt gotten bigger because I assume if I lost all pressure, I would have absolutely no braking power.
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07-15-07, 04:28 PM
#10
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if you dont have a lot of money then you shouldnt be wasting it on the vacuum hose when the master is your problem. replacing that hose WILL NOT stop the pedal going to the floor. If the hose were bad then the engine would not idle and the brake pedal would be VERY hard. I.E. the pedal would only travel an inch or so when pressed
replace the master, then worry about the hose
replace the master, then worry about the hose
07-16-07, 08:45 AM
#11
if you dont have a lot of money then you shouldnt be wasting it on the vacuum hose when the master is your problem. replacing that hose WILL NOT stop the pedal going to the floor. If the hose were bad then the engine would not idle and the brake pedal would be VERY hard. I.E. the pedal would only travel an inch or so when pressed
replace the master, then worry about the hose
replace the master, then worry about the hose
I never thought a bad master cylinder would cause your idle to drop when you hit the brakes, Ive always associated a problem like that with the brake booster because it has a vacuum hose going to it and loss in vacuum pressure can cause a loss in idle , correct ? So maybe both the hose and the master cylinder are bad.
Can someone please tell me where to purchase a new hose for the brake booster ... I already know plenty of places to get a master cylinder.
Last edited by wthdidusay82; 07-16-07 at 08:52 AM.
07-16-07, 08:57 AM
#12
A master cylinder can fail in different ways. It doesn't have to be leaking to fail. When the rubber cups inside wear or become torn, the fluid will "bypass" the cups and move back and forth inside the master cylinder instead of being forced out to the calipers. That's why I asked if the fluid level was going down. If you're not losing fluid and the pedal goes to the floor, the master cylinder is "bypassing" and needs to be replaced. And before you buy a vacuum hose, disconnect the damn thing from the booster and see if you have a strong suction with the engine at idle like I said earlier.
07-16-07, 10:13 AM
#13
A master cylinder can fail in different ways. It doesn't have to be leaking to fail. When the rubber cups inside wear or become torn, the fluid will "bypass" the cups and move back and forth inside the master cylinder instead of being forced out to the calipers. That's why I asked if the fluid level was going down. If you're not losing fluid and the pedal goes to the floor, the master cylinder is "bypassing" and needs to be replaced. And before you buy a vacuum hose, disconnect the damn thing from the booster and see if you have a strong suction with the engine at idle like I said earlier.
Whats the suction have to do with anything? If the hose is bad it needs to be replaced, i wont be replacing the hose or the master cylinder until I have the money.
Where can I purchase the hose can anyone tell me ? I like to know things in advance.
The master cylinder I can purchase at advance auto or autozone.
07-16-07, 10:40 AM
#14
hose, buy it at the dealer. That's where I used to get mine before I joined the Mazdaspeed support program.
In my experiences, the hose is a common failure point for loss of pressure of the brake pedal, but, you can still stop the car relatively well. If your braking completely goes away, then i would think it's the master cylinder.
Most people associate loss of pedal pressure without loss of fluid as a master cylinder problem. The mazda vacuum hose can also fail and the symptoms are quite similar. It's happened to me twice already.
My suggestion is, buy the hose, because it's cheaper than buying a new master. Bleed the brakes and then test the pedal. If it's still soft, then you know it's the master cylinder.
With any hydraulics (brake or clutch cylinders), every tuner I've been to has turned me away from using aftermarket or discount parts store rebuilt cylinders. They always seem to come with nasty problems which then makes you buy another cylinder. They whole-heartedly recommend the mazda cylinders. I've never had problems using them (them being official mazda hydraulics), but I am trying an aftermarket clutch master and slave now. However, I'm buying from the company that makes them for mazda so while they may not be official mazda parts, they should be the same functionally, at least, in theory.
Oh, and btw, the idle dropping is the consequence of "POWER" brakes. You put more load on the electrical system until the BAC valve compensates by letting more air in and increasing the idle. If your BAC valve is bad, then the idle will just stay down.
In my experiences, the hose is a common failure point for loss of pressure of the brake pedal, but, you can still stop the car relatively well. If your braking completely goes away, then i would think it's the master cylinder.
Most people associate loss of pedal pressure without loss of fluid as a master cylinder problem. The mazda vacuum hose can also fail and the symptoms are quite similar. It's happened to me twice already.
My suggestion is, buy the hose, because it's cheaper than buying a new master. Bleed the brakes and then test the pedal. If it's still soft, then you know it's the master cylinder.
With any hydraulics (brake or clutch cylinders), every tuner I've been to has turned me away from using aftermarket or discount parts store rebuilt cylinders. They always seem to come with nasty problems which then makes you buy another cylinder. They whole-heartedly recommend the mazda cylinders. I've never had problems using them (them being official mazda hydraulics), but I am trying an aftermarket clutch master and slave now. However, I'm buying from the company that makes them for mazda so while they may not be official mazda parts, they should be the same functionally, at least, in theory.
Oh, and btw, the idle dropping is the consequence of "POWER" brakes. You put more load on the electrical system until the BAC valve compensates by letting more air in and increasing the idle. If your BAC valve is bad, then the idle will just stay down.
07-16-07, 07:15 PM
#15
hose, buy it at the dealer. That's where I used to get mine before I joined the Mazdaspeed support program.
In my experiences, the hose is a common failure point for loss of pressure of the brake pedal, but, you can still stop the car relatively well. If your braking completely goes away, then i would think it's the master cylinder.
Most people associate loss of pedal pressure without loss of fluid as a master cylinder problem. The mazda vacuum hose can also fail and the symptoms are quite similar. It's happened to me twice already.
My suggestion is, buy the hose, because it's cheaper than buying a new master. Bleed the brakes and then test the pedal. If it's still soft, then you know it's the master cylinder.
With any hydraulics (brake or clutch cylinders), every tuner I've been to has turned me away from using aftermarket or discount parts store rebuilt cylinders. They always seem to come with nasty problems which then makes you buy another cylinder. They whole-heartedly recommend the mazda cylinders. I've never had problems using them (them being official mazda hydraulics), but I am trying an aftermarket clutch master and slave now. However, I'm buying from the company that makes them for mazda so while they may not be official mazda parts, they should be the same functionally, at least, in theory.
Oh, and btw, the idle dropping is the consequence of "POWER" brakes. You put more load on the electrical system until the BAC valve compensates by letting more air in and increasing the idle. If your BAC valve is bad, then the idle will just stay down.
In my experiences, the hose is a common failure point for loss of pressure of the brake pedal, but, you can still stop the car relatively well. If your braking completely goes away, then i would think it's the master cylinder.
Most people associate loss of pedal pressure without loss of fluid as a master cylinder problem. The mazda vacuum hose can also fail and the symptoms are quite similar. It's happened to me twice already.
My suggestion is, buy the hose, because it's cheaper than buying a new master. Bleed the brakes and then test the pedal. If it's still soft, then you know it's the master cylinder.
With any hydraulics (brake or clutch cylinders), every tuner I've been to has turned me away from using aftermarket or discount parts store rebuilt cylinders. They always seem to come with nasty problems which then makes you buy another cylinder. They whole-heartedly recommend the mazda cylinders. I've never had problems using them (them being official mazda hydraulics), but I am trying an aftermarket clutch master and slave now. However, I'm buying from the company that makes them for mazda so while they may not be official mazda parts, they should be the same functionally, at least, in theory.
Oh, and btw, the idle dropping is the consequence of "POWER" brakes. You put more load on the electrical system until the BAC valve compensates by letting more air in and increasing the idle. If your BAC valve is bad, then the idle will just stay down.
Ive put a aftermarket clutch slave, i think it may need more bleeding or something because the clutch doesnt seem to engage with the bite it should have on a launch into first gear and burns more than it grips and just doesnt seem right but eh its not that bad , its just hard for me to launch it hard and make the clutch engage how I want it to .. I have a centerforce dual friction clutch and a racing beat lightweight steelflywheel, clutch has about 12,000 miles on it.
As far as loss in idle .. my car has the BAC removed and I have a blockoff plate on it so maybe thats part of the reason, I'd maybe consider putting the BAC back on but I no longer have the part and Im not sure how the vacuum hoses are routed on it ( i also belive think some coolant hoses go on it too , im not sure but my tb has the tb mod done to it and cold start removed so that prolly complicates things more) ... just seems like a pain to reinstall and I dont mind the loss in idle but it seems like I lose alot of my idle...like if i rev it up it will stay at 1100 rpm , then it drops back down to 750 and is really bouncey...then if i hold down the brakes full pressure it will goto like 500 and barely be running.
Oh and any idea how much that hose costs ? Im going to try getting it as soon as I can depending on the price.
Last edited by wthdidusay82; 07-16-07 at 07:20 PM.
07-16-07, 09:05 PM
#16
yea....especially not beck arnley, i've had a bunch of defective rebuilt parts from them. Just get it from mazdatrix, they sell tokico which makes the OE Mazda cylinders. The aftermarket stuff just seems to be too finicky to justify their low cost.
Actually, I had a centerforce and a lightened flywheel. I could slip the clutch pretty easily, but you're right, there was no real bite. Then again, with my driving style, I rarely chirp gears. I've also heard that the centerforce clutches and the RX-7's aren't a good match; I remember reading something about that on this forum.
Without the BAC, the only solution is to raise the idle another 200-500 rpm via the idle set screw. Hehe, there's a reason why street cars are recommended to retain their BAC valve and their thermowax.
The one-way vacuum check hose is around 20 bucks, I think?
Actually, I had a centerforce and a lightened flywheel. I could slip the clutch pretty easily, but you're right, there was no real bite. Then again, with my driving style, I rarely chirp gears. I've also heard that the centerforce clutches and the RX-7's aren't a good match; I remember reading something about that on this forum.
Without the BAC, the only solution is to raise the idle another 200-500 rpm via the idle set screw. Hehe, there's a reason why street cars are recommended to retain their BAC valve and their thermowax.
The one-way vacuum check hose is around 20 bucks, I think?
07-16-07, 09:47 PM
#17
yea....especially not beck arnley, i've had a bunch of defective rebuilt parts from them. Just get it from mazdatrix, they sell tokico which makes the OE Mazda cylinders. The aftermarket stuff just seems to be too finicky to justify their low cost.
Actually, I had a centerforce and a lightened flywheel. I could slip the clutch pretty easily, but you're right, there was no real bite. Then again, with my driving style, I rarely chirp gears. I've also heard that the centerforce clutches and the RX-7's aren't a good match; I remember reading something about that on this forum.
Without the BAC, the only solution is to raise the idle another 200-500 rpm via the idle set screw. Hehe, there's a reason why street cars are recommended to retain their BAC valve and their thermowax.
The one-way vacuum check hose is around 20 bucks, I think?
Actually, I had a centerforce and a lightened flywheel. I could slip the clutch pretty easily, but you're right, there was no real bite. Then again, with my driving style, I rarely chirp gears. I've also heard that the centerforce clutches and the RX-7's aren't a good match; I remember reading something about that on this forum.
Without the BAC, the only solution is to raise the idle another 200-500 rpm via the idle set screw. Hehe, there's a reason why street cars are recommended to retain their BAC valve and their thermowax.
The one-way vacuum check hose is around 20 bucks, I think?
07-17-07, 08:57 AM
#18
you probably have a messed up TPS sensor. Those are expensive to replace. If the sensor's already gone, adjustment won't help it much. If you read Ted's site he's got a great write-up of how to adjust TPS. If you don't have thermowax, then you can even adjust it while the car is cold.
The brake pedal feels soft when you use the brakes, correct? Then when you pump it at a standstill, it gets progressively harder, until you try to use it again? which then becomes soft?
The brake pedal feels soft when you use the brakes, correct? Then when you pump it at a standstill, it gets progressively harder, until you try to use it again? which then becomes soft?
07-17-07, 09:22 AM
#19
Put the BAC system back in. Get help from a local RX-7 fellow...the parts are pretty cheap. Just need a new gasket, no vacuum hoses needed - you'll see what I mean when you remove the block-off plate, and two small diameter coolant lines. One coolant line goes from the back of the water pump outlet I believe, and the rear one goes to the rear iron. It is a bit of a hassle to remove the upper intake manifold/throttle body, but if you get some help it's not too bad. You can also just put the BAC valve in and not the coolant hoses and it may work rasonably well. As per the factory training manual (search - link is on the forum), the coolant helps stabilize the solenoid temperature - it passes by the valve, not through it. See below...if you remove the two phillips head screws, you see the coolant doesn't flow through the valve...
https://www.rx7club.com/attachment.p...4&d=1184456795
https://www.rx7club.com/attachment.p...4&d=1184456795
07-17-07, 10:22 AM
#20
you probably have a messed up TPS sensor. Those are expensive to replace. If the sensor's already gone, adjustment won't help it much. If you read Ted's site he's got a great write-up of how to adjust TPS. If you don't have thermowax, then you can even adjust it while the car is cold.
The brake pedal feels soft when you use the brakes, correct? Then when you pump it at a standstill, it gets progressively harder, until you try to use it again? which then becomes soft?
The brake pedal feels soft when you use the brakes, correct? Then when you pump it at a standstill, it gets progressively harder, until you try to use it again? which then becomes soft?
The brake pedal will feel fine initially when i hit the brakes, then it will become soft. i.e. I am hitting the brakes to slow down and the braking is good but becomes weaker and weaker depending on how much pressure I am putting on them and how long I am holding the brakes down with this pressure.
When I am at a stop at a light, the pedal will go down as far as it will go and the only way to get it to re pressurize is to lift the pedal back up completely. Pumping the brake pedal while the pressure is loss builds up no pressure, it just keeps hitting the bottom of the pedals travelling point on the floor.
As for the other post...putting my bac back in sounds like a major hassle the would probably actually be worth it in the end...I just really dont have the motivation to do it. If my braking problem is fixed and the problem with idle dropping isnt as bad after I fix the brakes, I wont see much need to put the BAC back on, but thanks for the info.
07-17-07, 10:31 AM
#21
As far as the Booster and Master Cylinder is concerned, what does the pedal do with the engine off? Does the pedal still go all the way in? If so, the MC is part of the problem, as much as the vacuum hose and/or the booster.
07-17-07, 10:40 AM
#22
ah....correct, in the FSM, there are three tests that you can perform to ascertain the condition of your brakes in the brake section. It's worthwhile to take a look. If your pedal is stiff when the car is off and soft when it's on, i would suspect it's the brake booster, most likely the vacuum check hose.
07-17-07, 08:10 PM
#24
That being said, your symptoms sound like a failing master cylinder to me. Hell, both the master and the vacuum check hose could be messed up.
07-17-07, 11:46 PM
#25
What I know is, after replacing master cylinders and boosters on cars for the last 31 years of my life for a living, is that if all this guy did was replace his vacuum hose, his pedal will still go to the floor. Then we can all hear him whine because he chose not to listen and still has to cough up the bucks for a master cylinder. I bet he still hasn't checked the hose for vacuum to the booster yet.
