Changed front pads and bled the system. Now the brake pedal pumps up like normal when the engine is off, but goes nearly to the floor with the engine on. Tried bleeding again - rear seemed normal, but very little fluid comes out of the fronts. On the road my braking power is greatly reduced and there is no front-end dive at all, even if I stomp on the pedal. My guess is that my helper put the pedal to the floor and ruined the seal on the front piston in the MC while I was under the truck working the bleeder valve. Tried all of the easy tests - pumped up brakes w/engine off, then started engine - pedal drops to near floor; pedal doesn't drop after shutting off engine and holding pedal down for a couple of minutes; 1st pump on th pedal still goes nearly to the floor after truck has been sitting all day, so it's holding vacumn. Anyone had anything like this before? Any other ideas?
Thanks in advance!
(p.s. Don't put 'Master Cylinder' into a search unless you've got lots of reading time.)

 

Yeah, always put a block of wood under the brake pedal while pumping or this can happen. I wouldn't rule out air in the lines just yet. You can get a power bleeder or similar and try that. If it still goes to the floor, start shopping for a MC. Sorry man!

 

Update - replaced the MC and all is well. Tore apart the old unit and couldn't believe how dirty everything was. I'm starting to think that the system flush I paid for a couple of years ago was not done properly. Overall, pretty simple job. The biggest time consumer was bench-bleeding the new unit.

For anyone else with an early '89 model - the MC is the same as the '88's and these are the only vehicles that used this unit. It can be very hard to find and very hard to convince 'professional' counter people that their computer is wrong. A different MC can be made to work but will require cutting and splicing the brake lines at the MC. Also, if your replacement doesn't come with the fluid level sensor and you can't use the original, don't worry. The p-brake light won't stay on all the time. Just cover up the old connector and be on your way.

 

Glad you got it figured out.

 

Yep, mine was a larger FJ80 MC and didn't have a fluid level sensor connection, never saw the p-brake light come on and it works great.