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Low brake pedal: Brake booster?

Old 10-15-11, 03:54 PM
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Low brake pedal: Brake booster?

Hey everyone,
I've been fighting a low brake pedal issue for a while now (2 years, actually). I have replaced all 4 calipers 2 times, inspected CLOSELY the hoses and replaced the soft lines with SS braided. I have also replace the master cylinder twice. I'm at a loss, so, could this possibly be the brake booster itself?

Also note, I have bled the system. Many MANY times, and I also had it bled by a Mazda dealership and one of the local race shops. I have a hard time believing after gallons of brake fluid we still have air in the lines.

-Mike
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Old 10-15-11, 03:59 PM
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it could be (air bubbles) trapped somewhere in the lines ... yes that could happen even if u bleed like 50 gallons ... ok maybe a gallon.

have u ever check the vacuum of the brake booster? just wondering.
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Old 10-15-11, 06:25 PM
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Get a vacuum house and place it on the nipple of the caliper. Then place the other end into some brake fluid( 20oz or so). Open the bleeder screw and have someone pump the breaks (20-30 times all four sides) and see if any air bubbles come out. Air bubbles should come out and break fluid will be sucked in instead of air. Repeat this for all four sides. Rear Right, Rear Left, Front right and Left right. The guys at Rotary Performance told me to do this. I replaced my rear caliper a few weeks ago and my pedal is weak. Just haven't gotten around to re- bleeding them yet.
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Old 10-15-11, 09:07 PM
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you can take some small pinch off plyers and pinch off all 4 brake lines. then press the pedal, if it is still soft you know its not the calipers and its the booster or master cyl.

the majority of soft pedals is caliper slide pins locking up or corrosion on the brake pads. Caliper slide pins can be re greased and cleaned off and brake pads can be sanded down. (not the material but the part that sits on the caliper. bracket.your pads should slide into the caliper bracket, you should not have to force them in.
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Old 10-15-11, 09:56 PM
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booster has nothing to do with a low pedal, if it was bad or had failed it would take alot of extra effort to stop the car
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Old 10-16-11, 12:55 AM
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Originally Posted by J5sense View Post
you can take some small pinch off plyers and pinch off all 4 brake lines. then press the pedal, if it is still soft you know its not the calipers and its the booster or master cyl.

the majority of soft pedals is caliper slide pins locking up or corrosion on the brake pads. Caliper slide pins can be re greased and cleaned off and brake pads can be sanded down. (not the material but the part that sits on the caliper. bracket.your pads should slide into the caliper bracket, you should not have to force them in.
While I do appreciate the advice, I don't think pinching off my stainless steel brake lines is a good idea...
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Old 10-16-11, 12:58 AM
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Originally Posted by gear_grinder View Post
booster has nothing to do with a low pedal, if it was bad or had failed it would take alot of extra effort to stop the car
Well, let me throw something else in here. I disconnected the vacuum line for the booster and took the car for a drive. The pedal is right up on top and never drops, just no assist (obviously).
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Old 10-16-11, 02:30 AM
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a booster cannot be overly powerful as it works off of engine vac. so a low pedal is impossible due to booster failure

it must be enough of an assist to easily compress the air in the system where the pedal feels low vs no assist.
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Old 10-16-11, 08:41 AM
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Originally Posted by gear_grinder View Post
a booster cannot be overly powerful as it works off of engine vac. so a low pedal is impossible due to booster failure

it must be enough of an assist to easily compress the air in the system where the pedal feels low vs no assist.
I changed out the booster this morning. Yes, it was the booster. The shaft on the output side was getting hung up under vacuum, never allowing it to return all the way. When there is no vacuum, the shaft moves properly, but with a slight hesitation on rebound.

Who would have thought?
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Old 10-16-11, 09:17 AM
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Originally Posted by N1XRR View Post
Who would have thought?
Pretty much anyone who had eliminated the other variables as you did.
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Old 10-16-11, 09:26 AM
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NEw booster here i come=D
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Old 10-16-11, 12:15 PM
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if only the pedal is low and everything else in check, just adjust the pedal...
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Old 10-16-11, 08:45 PM
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so the brakes were on when the pedal was low?? that seems like some important information we didnt get
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Old 04-10-12, 09:45 AM
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Originally Posted by gear_grinder View Post
so the brakes were on when the pedal was low?? that seems like some important information we didnt get
Not quite, the engagement point of the brakes varied from being on top (only 1" of travel) to nearly on the floor (3" of travel).

Once activated, they where consistent.

On track, this randomness means early braking because you cannot count on when the brakes will engage. On the street this very dangerous
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Old 05-16-17, 05:55 AM
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I had this same exact problem, I just replaced the hoses to the brake booster, and now my brakes are perfect. There is a check value in there that can wear out. Part number is FDY2-43-480.

Sorry to bring back a dead thread but it took me a while to figure this out, hopefully it can help someone else out.
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