1988 C-1500 brake problems

[Mikey48]

So I have replaced front brakes, back brakes and hoses. With engine off I have great pedal, start it and it goes to about 2 inches off the floor. Checked the booster check valve and it seems to be Ok, lets vacuum in and no air out. Did all suggested tests to the booster and it operates like it should. Any ideas, master cylinder, booster or maybe Rear wheel ABS? Thanks for any help.

 

[Macfluke]

I have a 89 C1500 that had crappy brakes and this cured it...

http://www.gmt400.com/threads/1990-c1500-abs-rwal-removal.11427/

I also replaced the 1" bore master with 1 1/8"... firm pedal and brakes that stop now...
not sure if it will work for you, but it did for me...


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[magimerlin]

Did you bleed all the air out after doing all that?.. If so how did you bleed It? Did you adjust the rear so they are just barely rubbing the drums(very little drag)

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[Mikey48]

Thanks for the help guys. Will bleed again and decide from there. It is weird that it happens with the engine running and the power assist only. I will keep you updated. I'm thinking about replacing the master cyl too.

 

[Mikey48]

So I bled the front, back and the master cylinder. It is better but not ideal. It was not the ABS because I discovered it was by bypassed but not removed. The wires are still connected. I feel replacing the back shoes and self adjusters made a difference. I am going to replace the master cylinder and maybe that will make me happy. Remove the rubber cap off the porportional valve and it moves toward the front when the brake pedal is pushed down, not sure that is OK.

 

[Oldblue98]

So I bled the front, back and the master cylinder. It is better but not ideal. It was not the ABS because I discovered it was by bypassed but not removed. The wires are still connected. I feel replacing the back shoes and self adjusters made a difference. I am going to replace the master cylinder and maybe that will make me happy. Remove the rubber cap off the porportional valve and it moves toward the front when the brake pedal is pushed down, not sure that is OK.

Just saying, did you start bleeding at the right rear,left rear,right front left front in that order ? Should start at farther wheel and work your way back to wheel closest to master cylinder. Best way to leave no air behind. And put a 2x4 under brake pedal as not to travel brake rod to far.
And booster will be firm and hard usually with it not running. If no air in lines make sure rears are adjusted up to slight drag... And then if no luck move to master cylinder

 

[Mikey48]

That is how I did it. On to the master cyl.

 

[Mikey48]

The new master cyl did it, have a great brakes. Old master cyl was 1 1/4 inch, new one 1 5/8 inch. On to the next project. Thanks for all the help.

 

[Schurkey]

Remove the rubber cap off the porportional valve and it moves toward the front when the brake pedal is pushed down, not sure that is OK.

The proportioning valve has no rubber cap. Proportioning valves affect ONLY the rear brakes.

The metering (holdoff) valve has a rubber boot. Metering valves affect ONLY the front brakes.

The proportioning valve, the metering valve, and a safety switch are all incorporated into the Combination Valve.

Yes, the stem will move forward when the brakes are applied. If you are pressure-bleeding at moderate pressure, you might have to push the stem in to get the front brakes to flow fluid. I've never needed to, because I pressure bleed at very low pressure (15 lbs or so.)

 

[Steve A]

Just out of curiosity, where/what model did you find a master cylinder that had a 1 5/8" bore?