I had same problem today and fixed it...
Brakes stopped truck ok but pedal felt mushy and holding firm pressure on the pedal it would go to the floor. First the red "BRAKE" light, immediately followed by the amber "ABS" light. If I turn off the truck, pump the brakes a few times in 10 seconds and restart the truck the lights would clear until I held the brake for 20seconds or more...
I replaced the master cylinder and bled all four corners until clear fluid came out. Still the identical problem. After reading up on it I was directed to investigate the anti-lock brake hydraulic module from an online fix site:
On the RWAL (Rear Wheel Anti Lock) module which sits 3" closer to the firewall than the proportioning valve, there is a rubber cap pointed horizontally right towards the master cylinder. Poke a paperclip thru the rubber cap about 1-1/2" until it touches the back of the RWAL accumulator piston. With key off, have a friend press and hold pressure on the brake pedal. If the piston moves, the service manual says to replace the RWAL. Mine moved, so the RWAL is supposedly bad. The only way for brake fluid to move the piston in the accumulator is for the anti-lock solenoid to be energized with battery voltage via the green wire on the "A" terminal of the connector... (OR if the solenoid valve is blocked open??, I thought.)
The leaking valve seat let fluid pass into the accumulator. The accumulator is spring loaded, so the "leak" doesn't go on the ground, rather it's collected in a spring loaded piston chamber and when you let off the brake pedal, the spring pushes the fluid back into the rear brake hydraulic lines & back into the master cylinder. Viola! Mushy pedal in spite of the following; brand new master cylinder, no air in system, fully bled system, NO visible fluid leaking under the truck.
My solution: My master cylinder reservoir was filthy with little black hunks of ??? in it. I suspected a chunk got caught under the solenoid valve seat during a hard stop yesterday, so with the key off/engine off I put 12volts to the green wire to energize the accumulator solenoid and repeatedly pressed the pedal to the floor and let it rest 10 seconds for a couple minutes. My theory was I could repeatedly fill and empty the spring loaded piston chamber of the accumulator and dislodge whatever might be blocking open the solenoid valve seat. SHAZAM! It worked...
If this problem comes back, I will do one of the following: Install a used RWAL module (new is big buck$) and powerflush all four corners again, or I will disconnect the rear brake lines from the RWAL module, connect them directly to one another, flush/bleed the system and effectively bypass the dang RWAL system.
...anyone need a used master cylinder that probably works just fine? It "only" has 246,200 miles on it.
Last edited by 25xs; 02-09-2011 at 06:28 PM.