Help With Proportioning/Combination Valve Identification

[cericd]

Got this prop valve from another member with the intention of having it rebuilt and using it on my 1970 C10 to which I've added factory style 1971 front discs and a booster to but I'm wondering if it will work for my application. Seems the factory ones I've seen have a rubber cap on the front. This one doesn't, just an indention and smooth metal. I'm told it came off a '71. Trying to figure out if I can have this rebuilt and use it or if I need to find another one. Here are some pics of the valve and the only pic I have of the truck it came from. Any ideas?

[Rust_never_sleeps]

That one has no metering valve, so it's from a 4 disk truck and won't work if you have rear drums, IIUC

https://help.summitracing.com/app/an...ering-valve%3F

[cericd]

I didn’t think GM offered a 4 wheel disc pickup in ‘71.

[dmjlambert]

Probably didn't, but a lot of parts swaps happen through the years, and even if it was physically taken from a 71 parts truck doesn't mean it was on that truck since 71. Between the time GM dealers ran out of spare parts for 67-72 trucks and the time restoration manufacturers tooled up, there was more junkyard shopping done to keep the truck going. When I bought my used combination valve I knew I had to chance it the valve came from a 1/2 ton 2WD truck because I'm putting it on a 1/2 ton 2WD truck. I have one that sort of looks right and may be right, but I don't know for sure.

[cericd]

I've see your post about your valve that you had rebuilt. I looks nice. I've also seen Keith Seymore's responses in a thread with the technical documentation. I don't see a part number on mine. If I post a WTB ad is there a way to determine that a used original valve is the correct one for a 2wd 1/2 ton swb pickup? I don't like the look of the aftermarket valves, they seem to have a reputation for leaking and they appear to basically be one size fits all other than choosing disc/drum or disc/disc.

[dmjlambert]

I don't know of a way to tell any more precisely than does it have all of the components you can see. The seller attributing what truck the valve came out of has some value, but obviously since the one you ended up with doesn't appear to have a metering valve that has to be taken with a grain of salt. I can imagine if you were buying a dozen of them and you knew what truck it supposedly came off of, about 9 or 10 of them would be the original and correct. But you'd have to get into collecting and testing and refurbishing these to find out, so that is kind of involved for somebody working on 1 or 2 trucks. It may be an option for somebody who wants to specialize in a component for a forum full of potential customers kind of like TBONE1964 used to specialize is instrument clusters.

To somewhat identify if it is a typical combination valve for disc/drum: The front of it has a metering valve with rubber cap. I don't know if the rubber cap is reproduced. If anybody, White Post may know. The combination valve I sent to them had the rubber cap on it. The middle section is the shuttle valve/warning switch and the electric connector is an indication it is there. The rear of it is the proportioning valve, and it is a round unit that screws in as far as I know, and it differs for different weight vehicles.

It is possible White Post would know if they ever change out the proportioning valve or if they could verify it is within spec shown on Keith Seymore's chart. I didn't think to ask for any testing results from their rebuild of mine and I don't know if they get into it that much. It may be possible for a mechanic to use some sort of test setup and gauges to verify the operation of the proportioning valve (and metering valve while their at it) to figure out if the numbers are in-spec for the 1/2 ton truck.

[cericd]

Thanks for that info. I've only got one truck so hopefully I can get by with just buying one more.

Anyone know if there were different valves for lwb and swb trucks? Or can I just get one from any 1/2 ton 2wd 71-72 GM truck? Also, does the fact that I have a Delco Moraine booster?

[pjmoreland]

I wonder if the primary difference between combination valves for different applications is the weight of the large spring that sits behind the port to the rear brakes. I've had my hands on two combination valves recently that both are Bendix 0411 castings, but one was a NOS unit spec'd for a K20, and the other I removed from a C30. The C30 spring is made of thicker gauge wire than the K20. They also appear to be color coded. One other difference between these two combination valves is that the K20 has a bleeder on the front, whereas the C30 does not. It would be interesting to see this same spring out of a Bendix combination valve from a C/10.

[pjmoreland]

Oh, and here's what the combination valve looked like on a 1972 C/10 I used to own. It had the rubber button on the front.

[dmjlambert]

That is interesting. It wouldn't surprise me if that spring was the difference. If I had known more, when I sent the combination valve to White Post, I would have asked them if they can inspect and test the proportioning valve and tune it to the specs in Keith Seymore's chart. It could be they have various parts on-hand including a bunch of springs, and can convert from one proportioning weight to another, or at least confirm by using gauges.

[dmjlambert]

In light of conversation below perhaps it is not for disc/disc, so disregard this: Chris, I meant to mention a possible scenario for you getting that disc/disc brake proportioning valve off of that 71 could have been because it had a rear disc brake conversion in its life, or it could be somebody just used it because it was what they found in a junk yard. There are a few people on the forum who like to do the rear disc brake conversion, somebody may be happy to buy that from you in the parts section.

[cericd]

Thanks for that extra info and pics Patrick. I'm surprised there isn't a bunch of info on the forum about this brake related part considering how much info there is from engine to specific hubcaps.

David, I'd gladly offer this one up on the parts board. I just wish I knew more about it so someone else doesn't end up with the wrong part. I knew it should've had the rubber cap on the front, I just wasn't paying attention when I picked it up.

Time to post a WTB.

[Kemp16V]

I have a 1971 C10 and the combination valve on it looks exactly like the one that you bought, it does not have the rubber cap on the front of it. I know that the valve on my truck is the original and has never been changed. I dont know if there is a difference but my truck has manual brakes. The stock disc brakes on the front and the stock drums on the rear.

[michael bustamante]

that might be the difference right there. power disc brakes vs manual disk brakes.. but then... i dont know enough about these to make any determination..
but my 72 conversion has the prop valve with the button and its a power assist brake system

[cericd]

Thanks for that info Kemp16V. That sheds some light on the situation.

Here's something else I ran across. Here's a post by Andy4639 which shows him using what looks like the same valve as mine with a booster.

Originally Posted by Andy4639

This is my 71 1/2 ton. The only time I have ever had a issue with the backs locking up is in a emergancy stop in the rain. On dry pavement they have never been a problem.

I know his truck isn't a stock example and that he has built his truck to be how he wants it. I'm just posting it as an example of my valve being used with a booster. I have sent him a PM asking about his experiences using this combination and what he knows about the valve.

Here's the thread his post is from: https://67-72chevytrucks.com/vboard/...83#post3197683

[dmjlambert]

That is interesting. I wonder if adding the metering valve was simply a small improvement after early 1971 trucks came out. It could have been added mid-year or for the 1972 model. Perhaps it is not important enough to worry about. I tend to doubt it was a power brake vs. manual brake thing. Metering valve is supposed to reduce nose diving during a stop.

[Rust_never_sleeps]

metering valve delays applying the front brakes slightly, so is considered essential in disk/drum systems.......but maybe it wasn't always thought of that way ;-)
I'm really curious now

[dmjlambert]

You know what would be interesting is to put that valve on a C10 and make movies from the side of it stopping from different speeds and with different aggressiveness of stop, and change out the valve to a one with metering valve and repeat the same movie making of same set of speeds and braking aggressiveness, and see if differences in nose dive can be seen in the videos. Perhaps use a GoPro camera mounted on a bracket that sticks out from the front bumper. I would also be interested to know if the driver can detect a different feel. The same proportioning valve and associated spring should be used for both combination valves by unscrewing and transferring it from one body to the other, because the weight or wear of the proportioning valve will change the feel and probably also affect nose dive to some degree.

[MikeB]

Originally Posted by dmjlambert

I wonder if adding the metering valve was simply a small improvement after early 1971 trucks came out. It could have been added mid-year or for the 1972 model.

If you have a lot of patience, you can dig through this website to find part numbers for metering valves. http://www.gmpartswiki.com/

I did it for a few items when I was converting an 82 C10 from 6-cyl/manual to V-8/auto, and easily spent hours digging through the various publications to find brackets, linkages, etc.

I don't know which term GM uses for a combination valve.

The AC Delco parts website is great for finding part numbers for items that are still available, like starters, alternators, switches, sensors, hoses, etc. https://parts-catalog.acdelco.com/acesCat.php

[cericd]

Originally Posted by dmjlambert

You know what would be interesting is to put that valve on a C10 and make movies from the side of it stopping from different speeds and with different aggressiveness of stop, and change out the valve to a one with metering valve and repeat the same movie making of same set of speeds and braking aggressiveness, and see if differences in nose dive can be seen in the videos. Perhaps use a GoPro camera mounted on a bracket that sticks out from the front bumper. I would also be interested to know if the driver can detect a different feel. The same proportioning valve and associated spring should be used for both combination valves by unscrewing and transferring it from one body to the other, because the weight or wear of the proportioning valve will change the feel and probably also affect nose dive to some degree.

That would be interesting for sure. Maybe the person who buys this prop valve from me can do that test.

I just want to replace this aftermarket one with one that works and looks correct but I'd sure like to see the outcome of that test. I haven't heard back from Andy yet but I have run across other images which seem to lean toward there being something to the 71-72 trucks with a factory booster got the prop valve with the rubber cap on front and the trucks without a booster got the one like I have.

[RichardJ]

When braking, weight is transferred from the rear to the front wheels. How much weight is transferred to the front is dependent on suspension design and varies with different vehicles. I would suspect wheelbase, spring rates and vehicle Center of Gravity all play a part.
Some vehicles squat on acceleration and some vehicles are more prone to nose-dive than others.

The Metering Valves come with different threshold pressures, ranging from 40 to 75 psi in the ones I've seen listed. That means with a 65 psi Valve, anything less than 65 psi coming from the MC the valve does not open and fluid does not move forward to the front calipers. If you have your foot barely on the pedal and maybe 50 psi coming from the front and rear MC ports. the front brakes are not working at all and the rear brakes are only slightly dragging with the 50 psi.
That works good at about 5 mph, say in a parking situation.
At higher speeds, if you do put your foot on the pedal you'll still have the 0 psi to the front and 50 psi to the rear, but the car won't know it. It'll probably slow down more simply because you took your foot off the gas.

At higher speeds, say 45 mph and say pressures are 300 - 400 psi, it will go right through that 75 psi threshold point, without ever knowing it is there. I.E. the brakes will function exactly the same whether or not there is a Metering Valve.

[MikeB]

Originally Posted by RichardJ

The Metering Valves come with different threshold pressures, ranging from 40 to 75 psi in the ones I've seen listed. That means with a 65 psi Valve, anything less than 65 psi coming from the MC the valve does not open and fluid does not move forward to the front calipers. If you have your foot barely on the pedal and maybe 50 psi coming from the front and rear MC ports. the front brakes are not working at all and the rear brakes are only slightly dragging with the 50 psi.

That's some good info. I never did 100% understand the term "hold off" and how the metering valve worked. It's simply "spring pressure that must be overcome", during which time pressure is immediately being applied to the rear brakes.

So, one valve delays pressure application to the front brakes, the other limits max pressure to the rears.

I always thought that combo valves should be tailored for a specific vehicle, but got into arguments with folks who claimed they are somehow self-adjusting to vehicle front/rear weight. Now I know that's nonsense, and yet tons of vendors sell brake kits with "one-size-fits-all" combo valves. You'd think there would be a choice of 3-4 of them, to at least get close to a specific vehicle's weight distribution.

[Keith Seymore]

Originally Posted by MikeB

That's some good info. I never did 100% understand the term "hold off" and how the metering valve worked. It's simply "spring pressure that must be overcome", during which time pressure is immediately being applied to the rear brakes.

So, one valve delays pressure application to the front brakes, the other limits max pressure to the rears.

I always thought that combo valves should be tailored for a specific vehicle, but got into arguments with folks who claimed they are somehow self-adjusting to vehicle front/rear weight. Now I know that's nonsense, and yet tons of vendors sell brake kits with "one-size-fits-all" combo valves. You'd think there would be a choice of 3-4 of them, to at least get close to a specific vehicle's weight distribution.

You are exactly correct.

There was a specific meter/split/slope for nearly every model/gvw/wheelbase combination.

K

[cericd]

I'm sure from the factory, the prop valve was tailored to the specifics of the truck but today, the aftermarket prop valves don't list these specs and seem to fall into two categories, disc/drum and disc/disc applications. I'm trying to get the proper OEM prop valve and have it rebuilt to factory standards.

Keith, would you happen to have documentation that shows what meter/split/slope I would need for my application. I am trying to keep my truck as stock looking as possible so there aren't any suspension/big brake mods to take into consideration.

My truck is a 1970 SWB C/10 with a 307. What other info would you need to look up the meter/split/slope specs for my truck? Thanks.

[Rust_never_sleeps]

I started a thread on combo valve variations and was provided with some good data before I saw something shiny and went another direction.
There were only a few specs for valves, so somewhere between "generic" and "tailored"

https://67-72chevytrucks.com/vboard/....php?p=9220460