Hey, I already posted this, but in the wrong section (sorry guys. Be nice, I'm new here)

The truck in question is a 1987 D100, 2WD, Slant 6, Auto, LWB.

The symptom:
When you stomp the brakes even relatively hard, they bite as good as they ever did and she stops on a dime, but if you are gentle on the brake pedal as if rolling up to a stop sign, the pedal slowly drops all the way to the floor and you have absolutely no braking power. It'll barely stop if you don't let up and pump the brakes.

The "I tried":
I've had this truck a long time and it sits a lot. Every now and then if I let it sit for a month or so with the parking brake set the rear brakes will lose their adjustment due to a weak brake spring that holds the lock against the adjuster wheel, but I already tried readjusting them and it had absolutely no effect this time. It acts like a slow leak is pushing the fluid out on gentle stops, but there are no leaks anywhere and the reservoir is still full. I'm starting to lean towards a failing master cylinder, but not positive enough about it to go drop money on a new one just yet. Any ideas? Any help would be appreciated. Thanks a million

 

Have you bled the brakes? Could be air in the system...

 

I let them gravity bleed a bit, and seemed good. I just pulled the master cylinder loose and the seal is bad on it, fluid was leaking into the booster. Gonna replace it and hope for success. Between this and removing the fuel tank yesterday to replace the fuel lines I'm tired of wrenching on this truck this week. Lol.

 

You need a new Master Cylinder. Good brakes when you stomp them, bad when you ease into them. Fluid leaking past the seal on the piston.

 

Yup. One new master cylinder and a whole lotta bleeding (rather difficult to do solo) and the brakes are back to their 'throw you through the windshield' selves. Thank goodness, I'm tired of workin' on this thing this week. Lol.