Ok, truck is 77 F150 4x4 driven pretty regularly since new. I replaced the master cylinder because it was leaking brake fluid from the seal. What will the booster exhibit if it is bad? I have noticed when I push down on the brake pedal it gives a quiet "pshhh" sound if that makes sense. The pedal also goes to the floor especially after the truck first starts.

 

RE bleed your brakes. Keep bleeding them until the pedal stops going to the floor. Did you bench bleed the part before you installed it? Air gets trapped in there preventing the brakes from working properly. If you can get the master cylinder level without removing it, you can bench bleed it still installed. Either you can get a kit that has fittings and hoses that direct the fluid back into the master cylinder, or plugs that simply plug the ports off. Pump the pedal slowly, have a helper check for bubbles. Stop when they are gone. Then re-bleed the system.

 

I used to have to pump my power brakes , finally after 31 years replaced it and now I barely hit the pedal and stops great , new booster , master cylinder , drums , rotors , etc , there is a pressure test that can be done on the boosters from the brake pros , makes a huge difference when it is working right , good luck .

 

I don't recommend bench bleeding in the truck even if it's level because the booster rod cannot always push the cylinder in far enough to properly bench bleed the MC. My 2 cents. To the OP - I agree with MB, sounds like air in the lines. The "pshh" sound is normal. If your booster is bad, your engine will run pretty rough because it'll have a vacuum leak. Did you bench bleed the MC and properly bleed the rest of the system?

 

I did bench bleed the new MC, however I have not done it at the wheels yet. I will try and give that a shot this weekend to see if it comes out of it. Thanks for the replys all.