Proportioning Valve Leaks on '74 K5

[my kids took the truck]

The proportioning valve leaks on my '74 K5 - why and how do I fix it?

Background: I have the usual story,the brake fluid was not changed every 5 years. Last year I replaced the master cylinder and rebuilt the calipers and rear cylinders. In the process the proportioning valve was activated to shutoff when I pressed the brakes without one of the calipers on the rotor. It reset and soon thereafter began leaking. It is a slow leak and over the last 12 months I have only had to add brake fluid twice and both times it was maybe a half-inch down in the reservoir. It is still a leak and it leaves a spot of brakefluid in my parking spot. So far all I have tried is to smack the proportioning valve with a ball hammer a few times to 'ring' it and loosen the crud. It stopped leaking for a few weeks but then leaked again. The brakes work fine and they bleed fine. The Brake light is off but I also don't know if it works - I need to take a multimeter and see if the proportioning valve is still activated. It sounds like the story is these fill with crud and rust from never getting flushed. Then if the proportioning valve activates it shifts the plunger into the rusted bore and tears the seals, then it forever leaks out the passenger side weep hole.

Before making this post I did some quick research - these are the posts I found relating to leaking proportioning valves:

• right here at coloradok5.com, this valve was not leaking it just had the light on - working properly, there was a pinhole leak
• another coloradok5.com, leaking valve, no solution found
• another coloradok5.com - dyeager535 said to get one from junk yard, poster did and that worked fine
• over at nastyz28.com, they found that the brass replacement proportioning valves all leak
• on the same nastyz28.com thread, in posts #21 through #33 the supplier of rebuild parts is trying to make a rebuild to stop leak on the OEM unit, looks like the exact same unit I have - he never posts if he assembles a rebuild kit
• at chevelles.com a parts supplier posts here and defends the brass replacement valves and claims 1% of them leak and come back
• the Pick-n-pulls in my area has three C10 trucks and no blazers - I could go grab a proportioning valve from a C10 and hope it is the same internally - if I knew the range of vehicles the same proportioning valve was used on I could pull from a larger range of vehicles
• rebuild kits sold here https://www.musclecarresearch.com/ but not for '74 K5 proportioning valve - the guy in the nastyz28 thread is the supplier
• http://www.gmpartsonline.net/ part number does not exist and the online lookup does not have 1974 as an option
• http://parts-catalog.acdelco.com/catalog/stock_check.php part number does not exist

Based on the picture posted in nastyz28 thread, mine is an exact match externally, assuming they put the same proportioning valve internals in the '74 K5 as the '73 Camero then my PN is 1237944


My options:

• pretend it is not leaking and go on with my day (did this for 12 months without luck)
• smack it a few times with the hammer again and hope for the best - loosen all four bleed valves and stomp hard on the brake to flush.
• take it apart and look for matching o-rings at Napa on a Saturday (then hope it goes back together - the word is it cannot be reassembled without some unknown tool, per nastyz28 post)
• buy a brass replacement and hope I don't get a leaker (everyone sells them - typical is carolinaclassictrucks.com - but the consensus is they are all from the same overseas supplier)
• ask musclecarresearch.com if they know of rebuilt OEM units or put together a rebuild kit (sent them an email through their website)
• buy this one from ebay style ebay-n-pull
• hope to find a good core at pick-n-pull (maybe I can get some vortec heads at the same time and look for q-jet parts)

Any help you can provide is appreciated.

 

[kdo1958]

why not buy a new one?
http://www.carolinaclassictrucks.com/73-86-Chevy-Blazer-Proportion-Valve.html

 

[my kids took the truck]

kdo1958 the consensus over at nastyz28 is that the new ones are junk and will leak. there are quite a few threads out there relating to the new brass units leaking.

That said - Scott at www.musclecarresearch.com wrote back and suggested buying a new one, he said he was not able to make a rebuild kit. He said it is a Bendix valve. He does not know of anyone rebuilding them. He also advised that the threads might not match.

In defense of the new brass units, at chevelles.com a parts supplier claims 1% of them leak and come back

I will let this thread hang out for a week or two and see what the popular opinion is on proportioning valves

 

[dyeager535]

I only have the '85-88 parts manual, so this is at best partially relevant.

For 1985-1986, there were 7 different combination valves used. Yes, I know people call it a proportioning valve, it's not, but I digress. The manual part numbers are of course broken down by body style and brake option, but that just gives you an idea of what may be out there. I know there are physical differences, as the late-model Impala SS guys complained that GM used the same combination valve for disks as they did drums, and the drum ones didn't work well with the disk brakes. You might look around, at one time there was a post on how to modify the SS combination valve to work better, but I've not searched it out in a long time.

If I've had one of these things apart, it's been so long I can't remember. Unless someone else has done it, tearing two that appear the same apart, and looking for the differences would be the only way to know if you could make one out of two of them. The body is brass, should be no issue with it alone, just the seals, fittings, and anything that might be drilled differently, but perhaps those are things that could be changed over from a new one.

At the link in my signature, I kind of think they have the parts manual for previous years. The combination valve in my parts manual is in Group 4, probably is in the earlier ones as well. (4.690 to be exact).

 

[my kids took the truck]

wow - talk about parts book and service manual repository

digging through those right now

found it, combination valve, CH5

"The combination valve is just what its name implies. The metering valve, failure warning switch and proportioner are "combined" into an assembly which also serves as the front junction block."

• "The Metering Valve "holds-off" front disc braking until the shoes of the rear drum brake contacts the drum." aka, pressure overcomes brake shoe spring tension
• "If the rear (or front) hydraulic system fails, the pressure of the good system forces the switch position to the right." contacting the electrical circuit and lighting the brake light
• "The proportioner part of the combination valve reduces the rear brake pressure and delays the rear wheel skid."

Here is the key advice on Ch 5 Section 8 of the 1974 Light truck Service Manual
"The combination valve is not reparable. If a defect is found in any portion of the valve, the complete valve assembly must be replaced."

I cannot find the part# nor combination valves in section 4 or 5 of the '73 - '78 Light Truck Parts Book

My options are less now

• smack it a few times with the hammer again and hope for the best - loosen all four bleed valves and stomp hard on the brake to flush.
• a brass replacement and hope I don't get a leaker (everyone sells them - typical is carolinaclassictrucks.com)
• this used one from ebay style ebay-n-pull or hope to find a good core at pick-n-pull

My combination valve is leaking out the 'rear outlet port,' I don't see anything about a weep hole, and that means it is the o-ring on the proportioning nut needs replacement or I did not get my rear brake line tightened properly. The question now is can I remove the proportioning nut and change that o-ring without much effort.

 

[dyeager535]

Never tried to take one out, but being a threaded fitting, hopefully it's steel and the brass will let it go easily.

Problem I've had with automotive o-rings is that there are some "one-offs", so you may find that it's not something you can buy, or is hard to find. Power steering and fuel line o-rings are like that.

Try taking the brake line loose, and blowing out/cleaning out the seat for the brake line flare. My oil cooler setup leaked since the day I installed it (or maybe a bit less lol), I finally got sick of it, pulled it apart, and noticed that some grit had gotten on the sealing surfaces. Wiped them clean, reassembled, problem gone.

Apparently a sign of the times, but by my 1990 manual, GM wasn't even listing transfer case or rear axle disassembly/reassembly instructions, so pretty obvious that as time went on, it was determined to be less cost effective to repair than to replace.