brake pedal has no pressure

I have a 1975 Chevy k10 350 I have bleed the brakes multiple times had some pressure moved it then lost all pressure. Pedal now sinks to the floor. I have noticed when I press the brake pedal I can hear a slight hissing noise that sounds like it’s coming from the vacuum valve booster or vacuum line. I think I’m going to just replace the whole thing but I’m 15 and trying to save all the money I can.

Tycen said:

I have a 1975 Chevy k10 350 I have bleed the brakes multiple times had some pressure moved it then lost all pressure. Pedal now sinks to the floor. I have noticed when I press the brake pedal I can hear a slight hissing noise that sounds like it’s coming from the vacuum valve booster or vacuum line. I think I’m going to just replace the whole thing but I’m 15 and trying to save all the money I can.

Click to expand...

Fill the reservoir have a friend pump the pedal,while you look underneath for leaks all along the bottom of the truck,and behind each wheel.Any leak? Fix the leak . No leaks found? Buy a combination valve centering tool and a master cylinder. A booster will not make your brakes go away it will just make them rock hard.If you have a leak to the booster a bad one way check valve or leaky booster,by all means change it,but those will not make your brakes go away. A wet streak behind the master on the front of the booster is good indicator of a bad master cylinder.

If you can't see any active leaks, you may have a problem with the master cylinder. Either defective or needs bleeding. Did they ever work for you or just go out? Is it a project you just got and had no brakes?

All good suggestions.

If the seal is torn or damaged inside the master cylinder it will not seal against the cylinder, and the pedal goes to the floor. Seal often gets damaged while bleeding an old master cylinder that has built up a ridge inside. Ridge is buildup where the seal has stopped moving for many years, but pushing the pedal to the floor pushes the piston past the ridge and damages the seal.

Welcome Tycen!
If you bleed the brakes and then not physically losing fluid somewhere, ie even if you can’t find the leak, the level in the mater-cylinder goes down, then find the leak and that is likely what is re-introducing air into the system.
If the fluid level stays the same but the pedal won’t hold, likely the mastercylinder is bypassing fluid inside which is highly probable.
The rest of the advice is solid here also.
Booster leak wont make brakes soft they will just not have power assist. If the leak is bad I’d suspect it runs rough/schitty like it has a vacuum leak. If it runs smooth then you can diagnose the hissing later imo.
Hope this helps.
Master is the likely culprit if not losing fluid on the ground.

Blockage in the master cylinder OR brake booster has ruptured the diaphragm.

3 Ton said:

Blockage in the master cylinder OR brake booster has ruptured the diaphragm.

Click to expand...

Both of these will cause a high hard pedal.

It can leak fluid out of the master cylinder into the booster with no sign of an external leak. I'm in the process of this repair as I type this. Also inspect your rear wheel cylinders. When they leak they'll contaminate your rear brake shoes and you won't really notice until it's usually way too late.