Combination valve

[idlook2]

I have a feeling that the proportioning valve is only for non-abs equipped trucks. Most of the trucks in that vintage had the Kelsey-Hayees RWAL brakes and have a combination valve. I think the proportioning valve is used to "balance" the brake pressure front to rear but that's not needed with the ABS unit. I did see something like the part you're describing here:
89-94 Chevy Truck Brake Proportioning Valve - C/K 1500-3500 (carolinaclassictrucks.com)

Hope that helps. It's really expensive new so maybe a wrecking yard is your best bet. Also, maybe an 1987 truck will have it or other non-abs truck.

That new valve looks exactly like the one I purchased from Southwest Performance Parts. I would guess that all of these aftermarket valves are identical and just being re-branded and sold by various distributors.

The machining quality of the valve in person is not great and it leaked almost immediately out of the electrical connector. I would recommend staying away from these aftermarket valves.

 

[idlook2]

I have the "no proportioning valve" model in my 1988, and it has RWAL, so that's not it either. I suspect it has to do with LD vs HD brakes difference in the 88-91 reg cab models, but I haven't confirmed that.

You might be onto something. My 1990 has the “light duty” brakes and included a proportioning valve along with the rear wheel abs system.

Does your truck have the “heavy duty” brakes?

 

[delta_p]

Some of the trucks used a height sensing proportioning valve. It would have been separate, in the rear brake system, and mounted down by the rear axle. It would've sensed the the chassis height above the rear axle during braking. They wouldn't have put two valves in series one at the MC and one at the axle.

 

[idlook2]

Some of the trucks used a height sensing proportioning valve. It would have been separate, in the rear brake system, and mounted down by the rear axle. It would've sensed the the chassis height above the rear axle during braking. They wouldn't have put two valves in series one at the MC and one at the axle.

Interesting, I’ve never heard of that. Got a picture?

 

[Gibson]

NAPA lists this part for a 1990 C-1500, "MMC-M552"
It has the plastic/rubber cover for the little stem that moves out.
Their is a tool that was used to hold the stem from moving so that you could bleed all the brakes more easily.
One of the jobs of the combo valve is that it doesn't allow pressure to the front brakes until enough fluid has been pushed to the rear drums to expand the shoes.
When the shoes are firmly against the drums, then the pressure builds and the combo valve allows pressure to the fronts.
The deal with holding that stem with the tool is so that you can bleed the brakes without having to build-up much pressure in the system.
All of the disc/drum systems have "proportioning" built into the system.
The RWAL is NOT for proportioning, it only senses wheel lock-up, actually not wheel lock-up, it senses driveshaft lock-up.
Their WAS a proportioning valve mounted to the frame, that used a lever to the rear axle,, a heavy load, as it compressed the springs would move the lever and allow more effort to the rear brakes, but I don't know if they used it on the 1500s.
Edit,, the above post already mentioned the "height sensing" valve.

 

[someotherguy]

I have a feeling that the proportioning valve is only for non-abs equipped trucks. Most of the trucks in that vintage had the Kelsey-Hayees RWAL brakes and have a combination valve. I think the proportioning valve is used to "balance" the brake pressure front to rear but that's not needed with the ABS unit. I did see something like the part you're describing here:
89-94 Chevy Truck Brake Proportioning Valve - C/K 1500-3500 (carolinaclassictrucks.com)

Hope that helps. It's really expensive new so maybe a wrecking yard is your best bet. Also, maybe an 1987 truck will have it or other non-abs truck.

All GMT400's have ABS of one form or another, whether RWAL or 4WAL.

Some of the trucks used a height sensing proportioning valve. It would have been separate, in the rear brake system, and mounted down by the rear axle. It would've sensed the the chassis height above the rear axle during braking. They wouldn't have put two valves in series one at the MC and one at the axle.

As far as I know, only an option on 3500HD's, possible they were also optioned on 3500's and I would suspect only on C&C models. Of all the HD's I've seen, so far have only seen ONE of those height-sensing prop valves - on my '94 3500HD.

No real clear pictures of it, though. It attached on a bracket just behind the side-mount fuel tank position. You can just barely see it in this pic; the bracket is the only thing in the pic that is still black, not rust-colored, LOL.. anyway, it has a trip lever on it that moves based on the suspension's ride height, connected directly to a variable proportioning valve to the rear brakes. It has an articulated link attached to the axle housing, IIRC.

You must be registered for see images attach

Another pic you can "sorta" see it in, between the side mount fuel tank and the framework for the wrecker unit my buddy and I were installing.

You must be registered for see images attach

Sorry all the pics suck. They weren't meant to feature the prop valve.. but the wrecker install.

You must be registered for see images attach

Richard

 

[Gibson]

As far as I know, only an option on 3500HD's, possible they were also optioned on 3500's and I would suspect only on C&C models. Of all the HD's I've seen, so far have only seen ONE of those height-sensing prop valves - on my '94 3500HD.

My '94 K2500 had one, although it was a HD version with a 9,200 GVW.
Maybe they only used them on the rigs that were over 8,600 GVW.

 

[arrg]

You might be onto something. My 1990 has the “light duty” brakes and included a proportioning valve along with the rear wheel abs system.

Does your truck have the “heavy duty” brakes?

No, mine has the LD brakes. So much for that theory!

 

[67GMC]

I have the "no proportioning valve" model in my 1988, and it has RWAL, so that's not it either. I suspect it has to do with LD vs HD brakes difference in the 88-91 reg cab models, but I haven't confirmed that.

I think you're agreeing with me. The ABS models have a combination valve (not a proportioning valve).

 

[idlook2]

I think you're agreeing with me. The ABS models have a combination valve (not a proportioning valve).

There might be some confusion with terminology. The combination valve has a metering valve on the front and a proportioning valve on the rear. Some trucks appear to have only the metering valve on the front with the rear made up of just solid aluminum, but the body of the valve appears identical.

All trucks have ABS.