brake booster?

check air hose from top driver side of plenium all the way to metal valve, then to booster. Hose might need replacing and/or the check valve needs cleaning.

 

Typical of a leaky diaphram in the booster. I would immediately check your brake fluid level; you may find it way down. Also bleed the brakes just a little and look for air bubbles. If you have them, don't do a complete bleed because it won't work and you'll be wasting your time. You'll need to replace or rebuild the booster. BTDT on my Lotus.

Ken

 

I agree with Steven. This past winter, I replaced all the hoses on my '78 GTS, and the vacuum hose running from the engine to the booster was not in great shape. If yours is original, consider replacing it. The only problem there is that to do it, you really have to move the left gas tank to snake the hose to the front of the car. It is not a difficult job, if you move the tank. And the check valve should be looked at also, as Steve said. Make sure you check the connections in the front of the car (remove the cover to expose the hoses).

Most of the time, the brake booster itself will be fine, but the hoses and connections get old and tend to leak. If you haven't done so already, consider changing all of the hoses, fuel lines and vacuum lines in the car. I did that last winter, and you can just imagine what some of those 30 year old hoses looked like. I was glad I did it ----> Peace of mind.

 

Changing 30 year old hoses is easy and worthwhile anyway; I hope it's that cheap and simple! My boosters are Girling and they last a long time, but are very difficult to rebuild correctly. Not sure what yours are...

Ken

 

i havw this same thing on my '84 308QV too now, the hissing continues as long as i keep the brakespedal aplied. i di think this is a diaphame or valve leakage inside the booster.
other suggestions.

 

How long does the line to the booster hold vacuum after you turn off the engine? If more than a few seconds your hose and check valve is probably okay. The hissing when the pedal is held down sounds like the booster is leaking.

How long it takes for vacuum to build up has a lot to do with throttle position. Closed throttle is a much higher vacuum condition than wide open. It is also possible that your engine is "down" on compression.

 

my though too, the hissingsound continues endlesly when hod the brakepedal down.

pretty sure the booster is at fault.

can anyone confirm?

 

Another test you can try if you have access to a vacuum pump.Disconnect the booster from the vaccuum hose coming from the engine.Hook up the pump to your booster through some pressure guages that read negative pressure and evacuate the air from the booster.If it holds vaccuum then the booster diaphram is ok.You can also test the hose in a similar way.Disconnect it from the booster and engine manifold.Plug up one end and connect the pump through guages to the other end and evacuate the air.If it holds a vaccuum then the hose is ok,but like the others have said if the hose is original and 30yrs old then it would probably need replacing

 

What does a faulty booster do to the pedal? I've had an inconsistent pedal... it's usually high and hard, but occasionally drops low... it doesn't have anything to do with heat buildup, and fluid flush and multiple bleeding hasn't eliminated the problem, which developed just recently. Normal hold down of the pedal has no creep, so it doesn't appear to be a problem with the master cylinder.

I've got Brembos on the front (about 2 years old) and stock calipers on the rear.