I have a 1998 chevy s10 pick up with a 2.0 and approx. 140.000.My brakes went out and I replaced all the way from the front to the back.I also replaced the wheel cylinders. I have bleed the brakes and have a good flow,but still don't have good pressure.The guy at the auto store said my proportion valve may need to be reset.I don't see one, and am stumped.

A couple of things could have happened here

Many times when there is a brake failure, it allows the brake pedal to go to the floor which the allows the master cylinder piston to travel into parts of the bore where it usually doesn't travel. This area usually has some corrosion or pitting in the bore which damages the rubber seals and ruins the master cylinder.

Another possibility is having the wrong wheel cylinders putting the bleeder on the bottom and trapping air in the cylinder that can't be bled.

The third possibility is still having air trapped in the ABS unit. This sometimes requires a scan tool to electronically bleed it internally.




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Thanks for the help, I appreciate it. I know its not the cylinders, but how would I know which of the other two it could be? I would think that if it was pumping it thru, that it wouldn't be the master cylinder, but you seem to know more about it.Thanks again for your help.

What do you mean by "good pressure".
Is it a problem with the pedal feel or the stopping power?
What does the pedal feel like?




78 Corvette Pace Car For Sale-18k mi orig-Florida

It won"t hold pressure.It seems okay when the truck is off, but the minute I start it, it is soft and goes to the floor.

You just don't have all the air out yet. You probably have some air trapped in the ABS unit and it may require a factory scan to to bleed it internally.




78 Corvette Pace Car For Sale-18k mi orig-Florida

It has me stumpped big time now. I went to move my truck to get another car out, and everything seems fine.I have breaks, they aren't soft, it's just like before the line busted.I have really appreciated your help, thanks,
Frank

Yes, the ABS unit bled itself out.
I have had GM vehicles that we have spent an entire day and gallons of fluid trying to bleed an ABS unit out and then suddenly it's bled with no warning.

Sometimes I take them out and slam on the brakes in a gravel surface and when the ABS activates, it bleeds itself out them.




78 Corvette Pace Car For Sale-18k mi orig-Florida