Quote:
Originally Posted by ErikW
They aren't vents. They are bleeders for the master cylinder. Sounds like the OP either has a bad booster or a vacuum leak.
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The bleeds are located directly above the brake line ports on the driver's side and have bleed valves in them. What the OP describes is something else, though by his description, I don't know what he is referring to besides the mounting holes in the ears of the flange.
OP, you have a brake booster vacuum leak. The booster is sealed and normally doesn't leak whether there is a master cylinder mounted or not. The mater cylinder does not see engine vacuum. You have a leaking brake booster. If the hissing is coming from the holes in the mounting ears, you leak is coming from around the mounting studs.
If the master does have holes in the bottom, I can't imaging what purpose they would serve. The body of the master cylinder seals tight enough that your internal booster leak is telegraphing the vacuum signal through those holes.
Short but sweet ... hissing, loss of vacuum and hard pedal is a leaking booster