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
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On Fri, 17 Aug 2007 21:38:28 -0000, snipped-for-privacy@gmail.com wrote:

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

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On Sat, 18 Aug 2007 18:45:59 +0100, "R. Mark Clayton"

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