1982 Ford F150 Hissing vacuum leak noise with no brake peda

Hi carolynwilson3. Welcome to the forum. This sounds typical of a leaking diaphragm inside the booster. Engine vacuum tugs on the diaphragm to apply the brake master cylinder, but atmospheric pressure is blocked from entering the backside so the diaphragm is held in the neutral position. When you push the brake pedal down a little it opens the valve allowing atmospheric pressure in. That allows vacuum to move the diaphragm which pulls the valve closed. That's why to move the master cylinder further, you have to push the pedal further. Every time you press the pedal down more, the valve opens, atmospheric pressure comes in which allows vacuum to move the diaphragm which closes the valve.

When you press the pedal to open the valve, you are hearing the vacuum pull outside air past the leaking diaphragm. Normally you will only hear that hiss while the pedal is moving down. The air flow will stop as soon as the pedal stops moving. Time to head to the salvage yard for a good used booster.

Still, the pedal should not go so close to the floor. I wonder if something came apart inside the booster. To be safe, have you checked for fluid leaks? A master cylinder leaking internally will cause a low pedal too but no visible loss of fluid.

caradiodoc

I have checked the entire brake system for visable leaks and havn't found any. I don't know how to check the master cylinder for an internal leak that isn't visable... but I assumed if there was an internal leak in the master cylinder it would affect which brakes (front or rear)actually engage.

I tested the vacuum on the booster as recommended by multiple researched sites and when I turn the engine on the pedal moves away from my foot towards the floor on it's own.. I understand that if this happens then the booster is working properly Maybe I don't understand the diaphragm seal correctly but I thought if the diaphragm was leaking there would not be enough vacuum pressure to pull the pedal away?

The only other thing that I can reply to is the air flow stopping as soon the pedal stops moving... With Engine off I pump pedal to remove reserve and resisitance pressure increases dramatically with each pump to the point that I can use both feet and only push pedal about 1/2 way to floor, which I thought means I have a good seal, with pedal in hold position about 1/3 way to floor I turn on engine and vacuum pulls pedal away from my foot towards floor with no problem... at this point I have not heard any hissing... I pull my foot away after about 30 seconds and pedal comes back up slower than expected and no noise yet... I put my foot on pedal and press towards floor the noise appears and there is no resistance at all until brakes engage about 1 1/2 from floor I keep pedal in hold position and noise continues but the flow speed decreases... I hold for 3 minutes with no change in volume or flow speed... I release pedal and noise continues flow speed increases a bit and push down again and repeat with same results... apon releasing brake pedal fully in stationary posiition the noise stops finally.. I don't hear vacuum or sucking of air though, the noise is air being pushed back out like a flat tire deflating only magnified 1000X.... I want to say that I don't think there is enough atmospheric pressure to move the diaphragm and close the valve properly or completely and if that happens the diaphragm would have more vacuum pressure and less atmospheric pressure resulting in a decrease of pedal resistance....

I am getting more confused....