brake problems on my cherokee

i have a 77 cherokee chief that i took out for the first time wheeling this weekend and all was well with the brakes except for steep uphill climbs, if i hit the brakes the pedal went to the floor and the brake light came on. whats up with that??? the reservoir was still full and there are no brake leaks. what causes this and how do i fix that?

no answers yet i see. i still havent figured it out. anybody???

[tincan]

Bad master cylinder

Drive downhill only.....Joking! Most likely is the M/C. Good time to do a complete check on all parts though.

[MouseMotor]

above are all correct. that is text book master cylinder.

while you are under there, it is time to start checking the lines for rust as well as leaking seals on wheel cylinders.

remember to bench bleed the cylinder as well. ideally you will put the master cylinder in a vice and let it sit, full of brake fluild. the brake fluid will start to leak down into the seals exc of the master cylinder and out the ports. once this has happened, bleed the remaining air from the master cylinder. it is important to allow some fluid to get into the seals before you start bench bleeding. if you just start pumping away with the cylinder dry you can damage seals. this may cause the master cylinder to fail right then and there, or fail prematurely.