I have a 94 S-10 Blazer with ABS. The brake warring light came on the other day and was interttermittent but has become very steady. The pedal is lower. I checked the wheel cylinders and found one rear cylinder seeping fluid. I rebuilt both rear wheel cylinders. The warning light is still on. The pedal seemed to force very little fluid from the bleeder valves. I then replaced the master cylinder. Still no help and the rear brakes are not working. So I dug out a manual and find there are a few circuits that control that warning light. I disconnected the brake pressure switch. The warning light goes out. This would indicate the proportioning valve ( control valve) Can this be true? I have never had a proportioning valve go bad before.

yes it can be true, they do go out but not often

You may need to find a shop with a scan tool so that the trouble code can be cleared. My brake/abs light came on, I replaced a wheel speed sensor but the light stayed on. after clearing the trouble code, the lights went off.

Thanks for the replies. I will work on this problem more this weekend. What I intend to do is remove the pressure control valve and attempt to clean it. The thought being, the fluid when I started this was almost black, so dirt is a possibility. I believe that the pressures are controlled by different size holes so a dirt or something could foul it up.

As far as airforceone's suggestion. I have a scan tool and it revels no error codes. I did not expect it to since warning light is the standard brake light not the ABS light.

Also remember that this truck has ABS. You will need to bleed the ABS module...which from what I remember was a royal PIA (pain in ass).
there were some special tools needed to depress the valves in the module and the proportioner valve to properly bleed the brakes.

Yes it looks like bleeding the ABS module will be a real PITA. Does anyone have suggestions on bleeding this module. Since the entire hydraulic system will be open air will be in all lines. I want to flush all the old brake fluid out and start fresh

Well it is fixed. The problem was the pressure control valve. I took the valve out. I noticed that each end had a 3/4' hex head so being as careful as I could be I loosened on end inside I found a plunger type of arrangement. I was removed and cleaned the rubber parts appeared in good condition so after cleaning I reassembled this end. The other end contained a different plunger and a fairly heavy spring. We need to detect pressure some how. After cleaning this end I assembled it back together. Installed the quasi rebuilt pressure control valve and bleed the system . A test drive show it brakes straight and true with no warning lights.