Faulty booster?

[C0V3R]

Friend of mine is having some trouble with their XB coupe. Their booster seems to be the culprit IMO, but looking for some opinions before throwing money/parts at the problem.

The booster passes the conventional leak tests - i.e.

- start engine, stop engine, leave it for 10min, check if there is still 2 applicatoins left
- Put foot on pedal, start engine, pedal drops

however there is a bit of a "weird spot" in the pedal. This spot is where the amount of assistance seems to change, goes from very mild braking to throwing you through the window. It is very difficult to modulate the pedal through this spot.

When the pedal moves through this spot there is an audible hiss from the booster. When checking stationary by hand, there is no hiss until hitting this spot, where the hardness of the pedal changes slightly. Pushing past this spot the hiss goes away and the pedal firms up a bit. If you keep the pedal in this spot we get an idle RPM increase (50-100rpm) and the o2 gauge shows a leaner mix (13.3->14.0 AFR) - i.e. 99% chance the booster is leaking at this spot.

What I am not sure about is why it suddenly seems to add extra braking force at this point, and if reco'ing it would fix it.

[RB30-POWER]

sounds a little like that rubber diaphragm reaction disc everyone talks about. (atleast in regards to the characteristic of the pedal actions)

although i have never heard of this accompanied by a vac leak as well.

can not elaborate anymore however in great or accurate detail.

someone else should know specifics.

if not a brake specialist would know one way or the other. (you would hope!)

[C0V3R]

Thanks heaps for that! I dug up this thread, http://forums.hybridz.org/showthread.php?p=1007706 and it sounds very much like the same symptoms.

[C0V3R]

Just found out about another test,

- engine on pedal depressed, turn off engine and check for pedal rising

Doing this test is ok anywhere except at the "spot". When I do it at the right spot, I get a noise like blowing bubbles into a milkshake from the booster for about 10-20sec.

Looks like its time for a reco.

[RB30-POWER]

that is a brilliant thread actually, shows it all rather clearly.

clears it up a little for me as well.

heard plenty of people talk about it, never had to deal with it. (thankfully i guess)