brake problem

[offroadjoe]

ok guys listen.. i got a 1988 gmc sierra 1500 with a 4.3... now my brakes are fucked the pedal goes right to the gorund and dosnt stop for shit.. this happened before and it was the master cylinder i went to the scrap yeard i got a new one... 3 months later the same problem just worse... now i said fuck the scrap yeard and i'm gunna go to canadian tire for a re-built one.. but first i need to no for sure if its the master cylinder.. ok now theres no leaks and the break fluid is always full so its not a line.. and if i pump the brake pedal it gets harder but not alot just enough to stop .. got any ideas it must be the cylinder or air in the lines i donno but i bleed them when i changed it? thanks guys

[kenny-1907]

If it was working after you changed it and now it has stopped , most likely it is the master cylinder. some things just don't pay to get from a scrap yard. Air in the lines would cause a spongy pedal and leaks of that amount to cause a pedal to go all the way to the floor you would think you would see, and a bad power booster causes a extremely hard pedal. Just bear in mind that you can get a faulty remanufactured master cylinder as well. Quite a few years back i used to own a 68 dodge dart and the master cylinder went on it so i went to crappy tire for a reman one. To make a long story short , the sixth one worked , all the others were bypassing as soon as they were bench bled and installed.

[Chevy-SS]

Master cylinders often can be tricky to diagnose, but one way we used to tell was to pump up and then hold pressure on the brake pedal. If the pedal would pump up OK, but would then slowly go down without losing any fluid, that meant the internal seals in the master cylinder were bypassing fluid. We would either rebuild it or get a new one.

I'm sure you know this, but you gotta bench-bleed the master cylinder real good prior to installation.