My truck is making a hissing noise when I hold the brake pedal down... sounds something like a vacuum leak. I suspect this means the power booster needs to be replaced, but I thought I'd ask for some other people's opinions before I plunk down $80 for a new booster. Thanks, Ed PS- front disc, rear drum, dual reservior m/c.. need any more info?
Check the hose from the booster to the manifold. Cheaper than replacing the booster then finding out it was the hose.
That's definitely the booster, but it doesn't necessarily mean it's bad. One of the trucks I drove for work last year would hiss LOUD at all times when pushing the brake pedal, but it worked just fine. I think it's the check valve that does it.
to find out if the booster is bad, have the car off, put your foot on the brakes hard, start it up (while still holding the brakes), if the pedal travels further when you start it is probably ok, there might be a leak but it will still have enough vac. to work.Get a second person to find out where the noise is coming from.
If the booster still works and you have done the tests the other guys have suggested. It is probably the sound deadener, foam around the pushrod. Boosters draw air in around the pushrod when you push on the brake and the differential pressure between the vacuum side (front) and the atmosphere side (cabin) is what gives you the****ist. If the diaphragm is ruptured you will get very little****ist. Likewise if you are getting a constant hissing noise the poppet vavle is most likely trash. If it only hisses when you push the pedal then the noise/dirt filter is gone.. Most remanufacutred boosters use a piece of imitation Jute, you know, the stuff you put under carpets or a piece of foam rubber like in your seats.
The booster functions fine.. it just makes a loud hissing noise when I step on the pedal. From whart you guys have said, it seems like the filter or the check valve. What is the funtion of the check valve (ie, where in the system is it mounted)? They're only $9 at NAPA so it may be worth a shot. Thanks, Ed
The check valve on the outside of the booster that has the vacuum line going to your carb or intake manifold is not what we are talking about. It holds residual vacuum in the booster when manifold vacuum drops. The poppet valve is inside the booster, connected to the push rod and is what makes a vacuum booster work. When you push on the pushrod with your brake pedal it lets atmospheric pressure into the booster. Having vacuum on one side (engine side) and atmospheric pressure on the other side is what give you the****ist. For a more detailed look at what is going on CLICK HERE For good general information about brakes look HERE The only thing I can tell is wrong is the piece of foam rubber on the intake has degraded or is missing.
Also check the rubber grommett that the hose from the manifold goes into. They sometimes split where you can't see it. Could be that.
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