95 S-10, Brake Master Cylinder Failure or Not?

My boy's 95 S-10. Since he got it, with the motor running the brake pedal is spongy & can be pressed to the floor although it will stop the truck. But with the motor off the pedal feels firm and stays high. We bled the brakes (Farthest away 1st). Still the same symptom. Still air in it or maybe a master cylinder?? I was thinking the master cylinder's bad but not sure because the pedal will stay up when the motor is off (But I been wrong before).

Thanks,
Gene

My guess, from what you described, is still some trapped air. Also, adjust the rear brakes, I am assuming here that they are drums, that is also another guess from your description.

Here's another check, remove the vacuum line from the booster and plug the line. Start the vehicle up and try the pedal and see if it stays up or goes down. You are checking for a leaky valve in the ABS that may not show up and give you a light.

Try the adjustment first, the vacuum line second, and bleed on the third try.

Let us know, Good Luck!

THANKS,
I did try the rear drum adjustment & it was fine. I even tightend them up a little more with no change. In my haste I have already replaced the master cylinder but waiting on a bud to show up to help me bleed it. I did bench bleed it before installing the new one. I'll try pulling the booster vacuum line & see what happens too.

When I pull the vacuum line, should the pedal stay up?

Thanks Again,
Gene

Yes, the pedal should stay up, if it goes down, then you are leaking in a valve in the ABS. Remember to plug the vacuum line you remove from the booster, otherwise, your engine may not idle if it does at all.

I pulled the vacuum line to the booster & the pedal even though a little stiffer did go to the floor. I bled it again with little or no improvement. But I did notice I still got air from each wheel. Another note I should mention is that I did not use a proprtion valve depressor tool on the proportion valve or the ABS pump. I am thinking I got air in the ABS pump, I got someone coming over in a few days to loan me the proper tool, pressure bleeder, & scanner.

Thanks,
Gene

Just an update & pass on what I found. After rebleeding again, I noticed on the ABS pump that one of the accumulator bleed stems sticking out further than the other. Using a blunt side of a pair of plies, a firm push & it popped back in. All seems well now. Don't know why but no more grabbing when tryng to stop at a slow roll and the pedal feels much better.

Thanks,
Gene