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'91 K5 Blazer - Proportioning valve stuck, ABS problems, and rear brakes won't bleed

pmgk

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I put this together in the hopes of helping "the next guy".

I had a chewed front rotor and the master cylinder level dropped enough that the brake warning light came on. Or so I thought. After replacing the front brake pads, brake rotors, and bleeding the calipers: My brake warning light was still on. I read a bunch about the proportioning valve/combination valve needing to be "reset" and I focused on that area. I noticed that every time the brake pedal was depressed, the pin under the driver's side rubber cap would extend. Holding the pin in with a J-39177 tool (or in my case, a fork with one tine bent up) resulted in fluid shooting from the pin bore. I tried the pedal stomp with all bleeders open and all bleeders closed: Plenty of fluid from the front two calipers, but no fluid from the rear brake bleeders. I cracked the hard line above the axle at the rubber brake hose in-case it was collapsed or plugged, still no fluid from the rears when pumping the pedal. I cracked the line before the ABS control valve: PLENTY of fluid. Cracked the line after the ABS control valve: no fluid.

Now that I had narrowed down the blockage, I turned my attention to the Kelsey-Hayes RWAL system. The rubber cap on the "modulator control valve"/"Electro Hydraulic (EH) valve" had no pin extended from it, so I tested the solenoids by energizing them with +12VDC. I unplugged both the 4-pin and 6-pin weather-pak connectors from the black ABS module next to the master cylinder. With test-leads (paper clips), I jumped the grounds together. Both are black wires and pin A on the two connectors. I next jumped the orange wire (battery circuit, pin "C" of 6) to the white wire (dump solenoid, pin "C" of 4) and then to the black/green
wire (isolation solenoid, pin "D" of 4). When energized, a click should be heard. In my case, the dump solenoid would click, but the isolation solenoid wouldn't click, even though it was pulling current.

Having diagnosed a bad isolation solenoid, I started planning an ABS bypass and the required parts. Just for ha-has, I whacked the ABS control valve with a ball-peen hammer. Just a couple pecks as there's not much swing room to get at it. Believe it or not: the isolation solenoid released and the rear brakes were then able to be successfully bled.

Judging by the state of the tar black fluid, this might have been their first bleeding. Now, the brakes are better than ever and I'm convinced that in the two years that I've had the truck, the rears never worked. Looks like there's a rear brake job in my very near future.

Enjoy this bit of anecdotal evidence as to why rear brakes won't bleed. Unrelated, if there is a brake fluid shortage in the North-East this summer, I had nothing to do with it.
 

1-ton

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I have a 1991 V3500 square body hat had different rear ABS system than your Blazer. It is just a mechanical valve that had a lever attached to the 14-bolt diff that sensed when the rear 14-bolt diff was rotating forward from hard braking, which engaged the rear ABS valve. I removed the entire valve, and by-passed it because it had gone bad causing the rear brakes to chatter.
 

ZooMad75

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Delete the crap. Period. You got it to bleed now. No guarantee it's not going to stick again. Parts are not available anywhere for it. I had the same exact problem with my 91 not wanting to bleed to the rears. I stopped well short of you troubleshooting. No jumping power to the solenoids or tapping it with a hammer. Just straight to the trashcan. Used lines and prop valve from a 87 I picked up from the boneyard and retrograded mine back to non-abs.

One thing to consider is to make the conversion, you will have to reflare the rear line with a new fitting to match up to the rear line output on the earlier prop valve. Other than that it's a basic bolt in operation.
 
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