spongy brakes BMW 740il

My e38 has spongy brakes. when you press down on the brake pedal the
brakes engage but when you release the pedal, it does not come all the
way up still engaging the brakes. you can release the brakes by
lifting the brake pedal with your foot. My brake guy thought it was
the master cylinder and replaced same. was better for a time but now
back to the same as before the replacement. went back to the brake
shop and he disengaged the power booster. hard to brake but the pedal
comes right back up after braking. he checked the vacuum pressure
which is ok. thinks it is the brake booster. maybe a leak in the
diaphram which will not allow the brakes to fully disengage. its an
expensive repair, does his diagnosis makes sense?

Thanks

On Fri, 17 Aug 2007 21:38:28 -0000, tim.primefinancial RemoveThis @gmail.com wrote:

>My e38 has spongy brakes. when you press down on the brake pedal the
>brakes engage but when you release the pedal, it does not come all the
>way up still engaging the brakes. you can release the brakes by
>lifting the brake pedal with your foot. My brake guy thought it was
>the master cylinder and replaced same. was better for a time but now
>back to the same as before the replacement. went back to the brake
>shop and he disengaged the power booster. hard to brake but the pedal
>comes right back up after braking. he checked the vacuum pressure
>which is ok. thinks it is the brake booster. maybe a leak in the
>diaphram which will not allow the brakes to fully disengage. its an
>expensive repair, does his diagnosis makes sense?
>
>Thanks

Yes but an EXPENSIVE repair? It could be that the air filter that allows air to
exit or enter the booster has become blocked.

When you press the brake with the engine running there is usually a slight
"hiss" when the booster is released. This is the air entering the unit to
equalize the pressure on both sides of the diaphram. If the air cannot get in
then the servo is still offering assistance to the braking circuit.

There is a valve on the end of the vacuum line to the servo - might be the
culprit or it might be the filter - usually around the push rod where it enters
the unit.

Probably a bastard to get at but I would go for the vacuum valve first then the
filter then another unit in that order.

However it might be that one or more of the flex hoses are separating
internally?

Hugh
--

Sir Hugh of Bognor

The difference between men and boys is the price of their toys.

Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen
hsg RemoveThis @h-gee.co.uk
Bognor Regis, W.Sussex, England, UK