07-16-2009, 04:38 PM
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Trained Professional
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Join Date: Feb 2003
Location: New Jersey USA
Posts: 12,225
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 Quote:
Originally Posted by Dragline
In this case, wouldn't the fluid accumulate only in the non vacuum side of the brake booster (aft chamber)? Wouldn't this only be drawn into the engine if the brake booster diaphragm also leaked?
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What can happen is that the vacuum in the booster can suck the fluid out of the brake master cylinder (M/C) through a leaking rear seal. So the fluid goes into the forward (M/C side) of the booster. This is the side of the booster that has the vacuum line but it is not the only side of the booster that has vacuum. When the brake pedal is released, the air valve in the booster opens an air passage which connects the front and rear booster chambers together. In this condition, there is equal vacuum on both sides of the diaphram. When the brake pedal is pressed, the air valve closes the vacuum passage and opens the rear chamber (brake pedal side) of the booster to atmospheric pressure. It is this differential pressure, atmospheric pressure on one side of the diaphram and vacuum on the other, that provides the 'power' assist. The 'whoosh' sound you hear when pressing the brake pedal is air entering the rear chamber to fill the vacuum.
Joe Papa,
Your symptoms indicate at least one (of two) failing piston primary seals in the master cylinder.
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Jim
Series 1 Euro 1750 GTV
Series 2 USA 1750 GTV
Series 3 Spider Veloce
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