Brake pedal long travel/hissing noise

[m3f_mrenshaw]

The brake pedal on my 98 M3 has a generally pretty terrible feel. Here are the symptoms:



- Long brake travel - the brakes bite when the brake pedal is about even with the accelerator pedal, with the car on or off. Pedal never goes all the way to the floor.



- Hissing noise - Air hissing noise under pedal depression, whether the car is on or off.



- Pedal will pump up and be firm at its highest point when the car is off, but not when the car is on.



- With the pedal held firm at its highest point after pumping it up with the car off, the pedal will sink to its lowest point (even with accelerator, not to the floor) upon starting of the car.



I think that the pedal has felt this way since I bought the car. However since I have owned the car, I have swapped pads, and bled the system. The previous owner installed an ATE brake master cylinder at some point.



To me it seems like maybe there is just air in the system, but the hissing noise is what indicates to me that something else may be going on, or maybe there is more than one problem.



Can someone help me diagnose the problem(s) or eliminate any of the likely suspects? I was prepared to replace several of the components at once, until I saw the price of the master cylinder and booster! :-)



Thanks!

[m3f_englishtom1596q]

hissing is normally related to the booster but what youre describing sounds like normal operations. pedal slowly dropping is normally a leak, air or master cylinder seals

[m3f_mrenshaw]

Could there be some sort of vacuum leak without the complete failure of the booster?


The car also has braided lines, so I doubt there is a any expansion going on. Also no exterior leaks that I'm aware of.


Does anyone have experience with the ATE master cylinder? It is the unit that is currently on the car, and I'm doubting its quality, perhaps wrongly, because it isn't OE, and if cost is any indicator...

[m3f_englishtom1596q]

i'm pretty sure ATE is the original manufacture. the booster could be failing but not completely failed yet but thats speculation. If there was a vacuum leak it would cause the engine to run rough. i'd honestly bet that your master has failed/is failing

[m3f_scoobiedoo2029]

ive had a check valve die and the hose connecting the check valve to the booster fall apart on an e28, car ran perfect otherwise. pedal was just hard as a rock.



my m3 has some hissing noise but no other effects, pedal is slightly softer then id like buts got 165k miles on it.

[m3f_mrenshaw]

I drove the car today for the first time in a while, and the front calipers seem to be seized up.


Is it possible that the gradual seizing of the front calipers were contributing to terrible pedal feeling?

[m3f_scoobiedoo2029]

Originally Posted by mrenshaw:

I drove the car today for the first time in a while, and the front calipers seem to be seized up.


Is it possible that the gradual seizing of the front calipers were contributing to terrible pedal feeling?

broken crap feels bad. but your symptoms could be a few things. id certainly start with calipers, rebuild kits are cheap and its not that hard to rebuild them, start there and change out and bleed your brake fluid and see where that gets you. itll be cheap before you go ahead an start looking into master cylinder/booster replacement. and it might fix your issue.


regardless the only thing a car needs to do is stop, running and turning are just cool, stopping is the important thing....

[m3f_Roland H]

If the booster was failing or failed, the pedal would be insanely difficult to push...


This is an issue with having air in the lines, probably due to a failure of some sort. Unlikely that the calipers have failed, or else you'd have a serious problem, willing to bet its the master.


I have a low miles master cylinder for sale. PM me if you want it.