# bad brake booster? I can't get a solid pedal...



## greenraVR6 (Mar 10, 2008)

I can't seem to get rid of my spongy brake pedal. I just installed a full stainless braided hose kit, mk4 rear calipers, and a new load sensing pressure valve, and my brakes are just as soft feeling as they have always been. I'm thinking it might be my brake booster cause the pedal is hard as a rock until I turn on the car and vacuum is applied. 

made this video cause this noise doesn't sound normal/good to me...


----------



## greenraVR6 (Mar 10, 2008)

:banghead: I want solid brakes


----------



## cuppie (May 4, 2005)

_*Brake boosters do NOT cause soft brakes.*_ 
All it does is use an air pressure differential to multiply the force that you apply to the brake pedal. If there's anything wrong (leaks) with the hydraulics, this will amplify the 'feel of that. 

Either they're not fully bled (how did you bleed them? Did you remember to raise the load-sensing valve before starting the bleed?), or the master cylinder is leaking. 

I'll bet money that your master cylinder is leaking - at that, I'll bet that the rear seal has failed. If that's the case, the vacuum in the front chamber of the booster will cause that seal to leak like crazy when the engine is running. 
Pull the master cylinder - any fluid in the booster?


----------



## EuroSportChicago (Jun 9, 2010)

cuppie said:


> _*Brake boosters do NOT cause soft brakes.*_
> All it does is use an air pressure differential to multiply the force that you apply to the brake pedal. If there's anything wrong (leaks) with the hydraulics, this will amplify the 'feel of that.
> 
> Either they're not fully bled (how did you bleed them? Did you remember to raise the load-sensing valve before starting the bleed?), or the master cylinder is leaking.
> ...


 +1


----------



## greenraVR6 (Mar 10, 2008)

I bled the brakes with a pressure bleeder a couple times, then with someone pumping the pedal. I then realized I needed to pull back the spring for the rear prop valve and found the whole thing was rusty and seized. Replaced it, bled with the spring pulled back and brakes are back to the same as they've been for the past 2 years, soft pedal and no fluid leaking or disappearing. 

What do you guys make of the air "wooshing" noise when I push my brake pedal? I can only hope that the master is bad cause they're only $40 new compared to a new $300 booster


----------



## cuppie (May 4, 2005)

The air noise that you hear from the brake booster is (to a point) normal - remember, it's a pneumatic device. Pressing the brake pedal effectively creates a leak, so that it can operate.  

If you're damned sure that the brakes are bled properly, and the pedal is still 'soft', then it's the master cylinder. 
That, or one or more siezed caliper slide pins (yes, I've actually seen this. Even saw a tech put a MC on a car to fix it! Sorry, didn't remember this one earlier.) Quick check - pull all of the brake calipers, and verify that the slide pins do all still move in the carriers. Fix/replace as needed.


----------



## germancarnut51 (Mar 23, 2008)

Bleed the brakes with the rear of the car on the ground to make sure that the proportioning valve is open. 

If a proper bleed with 1-2 quarts of fresh brake fluid doesn't firm up the pedal, then the master cylinder is shot.


----------



## cuppie (May 4, 2005)

But, that's kind of a pain in the a$$, trying to wiggle under the car.  
Rather than do that, I prefer to simply raise the left trailing arm with a jack. Does the same job.


----------



## greenraVR6 (Mar 10, 2008)

after reading what you guys have said and looking through the bentley manual again, I'm gonna buy a new master cylinder


----------



## EuroSportChicago (Jun 9, 2010)

greenraVR6 said:


> after reading what you guys have said and looking through the bentley manual again, I'm gonna buy a new master cylinder


 If the master cylinder was the problem, you would just have a "heavy" pedal. 

Spongy brakes is the #1 symptom of having air in your lines, and that would be easily explained how you just replaced the caliper and the brake lines. Bleed it thoroughly again, and use the Vagcom to bleed it with the ABS.


----------



## EuroSportChicago (Jun 9, 2010)

EuroSportChicago said:


> If the master cylinder was the problem, you would just have a "heavy" pedal.
> 
> Spongy brakes is the #1 symptom of having air in your lines, and that would be easily explained how you just replaced the caliper and the brake lines. Bleed it thoroughly again, and use the Vagcom to bleed it with the ABS.


 Oh and the brake booster is ran off vacuum, so of course the pedal will be hard while there is no vacuum applied (When it is off)


----------



## greenraVR6 (Mar 10, 2008)

EuroSportChicago said:


> If the master cylinder was the problem, you would just have a "heavy" pedal.
> 
> Spongy brakes is the #1 symptom of having air in your lines, and that would be easily explained how you just replaced the caliper and the brake lines. Bleed it thoroughly again, and use the Vagcom to bleed it with the ABS.


 What do you mean by "heavy" pedal? 

According to the bentley: 

- low pedal after bleeding is an indication of faulty master cylinder, 
-spongy brakes is an indication of air in lines or faulty master cylinder 
- pedal slowly falling to the floor with no indications of leaks in the system is an indication of a faulty master cylinder. 

I pretty much have all 3 issues (not so much the pedal falling to the floor, but it doesn't seem super solid under pressure). Gonna check the slider pins on the fronts and I know the rears are good cause I cleaned and greased em before installation. Will try to bleed them again and if I get the same results, I'll throw on a master. No need for vagcom cause it's a '90 gli...


----------



## EuroSportChicago (Jun 9, 2010)

By heavy, I mean that it is not assisted, its very hard to push it down and does not feel as smooth/nice like usual. 

As a precaution that could save you money, I would make sure it was bled properly. It will only take 30 minutes and might save you a couple hundred bucks. 

You still probably have air in your system.


----------



## greenraVR6 (Mar 10, 2008)

swapped out my master yesterday, bled the system and came up with the exact same pedal feeling :banghead: What is the secret to bleeding these brakes? Do you just have to bleed a whole bottle through each caliper or what? I've pressure bled them and used a helper to pump, ending with nothing but clean fluid coming out at each corner, and no luck :banghead:


----------



## cuppie (May 4, 2005)

A few things that help: 
- lift the proportioning valve, or jack up the left trailing arm (*required*.) At full droop, there will be little to no flow thru the rear brakes. 
- with the presure bleeder attached and flowing, press the brake pedal a few times, for each wheel, _with the bleed screw open_. Yes, you read that right.  Doing this greatly increases the flow rate thru the lines, helping to usher the air out. Don't pump" it, don't stab at it - simply smoothly stroke the pedal 4-5 times; then allow fluid to continue to flow for another 5-10 seconds. 
- Pressure is your friend. The M/C reservior will handle 25psi. Don't be the pansy who only runs the pressure bleeder up to 8psi.  
- Since you're running MkIV rear calipers on a MkII, you do realize that they're mounted upside-down, right? Before you start bleeding, you need to remove them, flip them upside-down, and block the pistons with a piece of wood - the bleed screw needs to be at the top.


----------



## greenraVR6 (Mar 10, 2008)

gonna have to try your bleeding technique...and my bleeders are at the top, you can see em just behind the upper carrier bolt


----------



## BiH (Nov 27, 2000)

its your MC, I had the same problem when I replaced my rears and put calipers, prop valve was rusted and frozen. I put a new MC and after everything it was soft, it worked sort of but not like it was. replace your MC, make sure you dont push it in while its dry or you will ruin the seal (thats what happened to me). bench bleed the MC, it usually comes with lines and fittings! mine are perfect now!


----------



## greenraVR6 (Mar 10, 2008)

I guess I'm just gonna have to spend some more $$$ and buy a new master...I hate dealing with brakes :banghead:


----------



## WeekendMechanic (Jul 29, 2013)

I am having the exact same problem when braking I have to push down hard to stop and even then there is reduced resistance is this a brake booster or master cylinder?


----------



## GTijoejoe (Oct 6, 2001)

cuppie said:


> _*Brake boosters do NOT cause soft brakes.*_


 An over boosted system will cause *soft *pedal feel. This is the reason when your car is off you stand on the pedal its hard as a rock, turn the car on and it will sink further to the floor. 

If your booster is broken structurely and deflects under load, this will also cause a soft pedal feel. 



EuroSportChicago said:


> If the master cylinder was the problem, you would just have a "heavy" pedal.


 Blown MC seals would result in very little resistence causing the pedal to feel light 

:sly: As someone who designs brakes for a living, there some poor advice/facts throughout this thread :thumbup: 


To the OP, more then likely you have a MC concern, but food for thought, when you change out calipers especially to newer models you can lighten up your pedal feel because of the type of seal grooves which are machined in the calipers. With fuel economy being on the minds of many OE's they tend to provide seal grooves which have higher piston roll back for drag purposes, these tend to lean in the direction of spongy pedal feeling.


----------

