mailbox1024
If you can pump the pedal back up and then the pedal sinks back to the floor, then you have a defective master cylinder. Check the cylinder to make sure you still have fluid filled inside of it. If it is low then you have a leak somewhere, but when a booster goes bad it feels very hard and no effort to stop the car. My bet is a defective master cylinder. If a garage installed it, the nit should be under warranty in full parts and labor, but if you installed it, the nyou can get a free replacement where you bought it. good luck........

john e
Sounds like the master cylinder

Dk
It sounds like it could be the Brake Booster. Providing your engine is putting out adequte vaccum. I would check the vaccum pressure first it would be the cheapest. If it is an 86 their are probably a ton of miles on it right. I would Check vaccum or have it checked (a shop probably wouldnt charge much for that) then go from there. Hope this helps

antiber
I agree with u, looks like it ur booster. With help from someone, u can know it; while u press the pedal, ask someone to listen at the booster, if the booster made a windy noise, then u need to replace the rubber of the booster. I believe, if the master or central is the problem, ur pedal will always go to the floor and u need to pump it to raise the pedal even only for a while.

PhilliesPhan
While the truck is not running, push the brake pedal and see if it gets hard to push. If it does not get hard to push then your master cylinder is bad. Otherwise, as someone else said, check the fluid in the master cylinder to see where it is at. If it is low then you have a leak and air in the lines.

avomatic
air in the brake system, check for leaking wheel cylinders. can also be caused by a leaking master cylinder. if no leaks are found, have brake system bled and refilled.