brake proportioning valve

[dooguy]

since my brake lite comes on when connecting to switch makes me think something stuck

[dmjlambert]

I would be interested in seeing a link to the cast iron valve on Classic Chevrolet website.

The local mechanic is OK for replacing them if not working right. That's exactly what the service manual says to do. :-)

[dmjlambert]

Originally Posted by dooguy

since my brake lite comes on when connecting to switch makes me think something stuck

Yes, probably. Question is can it be unstuck using some method like discussed here in the thread. And then test it to see if it comes on when it is supposed to. The procedure is in the service manual, here if you don't have a paper one.

[dooguy]

https://chevrolet.obsoleteandclassic...truck#page/104

I stand corrected
it says replacement style

[MikeB]

I've bled my truck's brakes (stock 71-72 master cylinder and prop valve) several times over the past 34 years, and never once had issues with the warning light coming on. In other words, the shuttlecock always returned to center without any intervention from me.

However, earlier this year I replaced the switch because it was simply worn out, causing the light to flicker. Problem solved! But now, after the truck has been sitting for five months during a wiring harness installation and other mods and repairs, the warning light decided to come on and stay on. I'll be taking it out in a few days for it's first drive since February to see what happens.

I do have a new prop valve from Inline Tube that appears externally to be an exact replacement, but I'd rather keep the stock one.

[RichardJ]

>>since my brake lite comes on when connecting to switch makes me think something stuck<<

The switch will stay closed (on) until YOU turn it off.

At this point, I haven't read that you have tried to bleed the brakes or opened a bleeder to try and move the switch.

In post #14 you wrote, >>when I stomp on brake pedal on gravel my rear brakes will lock.<<
There are so many variables there, that result is inconclusive. As "dmjlambert" is pointing out in post #24, a fixed rate proportion valve on a truck is a compromise. With a light load, the rate (reduction) will be too low and with a heavy load, the reduction is too high.

A new Combination Valve won't solve any problem until you know what the problem is.

[dooguy]

when pushing brake pedal and observing the front of valve body, a small pin pops out and then retracts when releasing pedal, but brake lite never goes out

[dmjlambert]

That front pin is the metering valve action. The metering valve delays brake action on the front disc brakes until rear brake drum fluid pressure starts to overcome the return springs. It is something separate from the shuttle valve that we have been talking about. The shuttle valve detects leaks in one of the two brake circuits and operates the warning light. The 3rd valve inside there is the proportioning valve, reducing the pressure going to rear brakes, since drum brakes do not require as much pressure as disc brakes.

[dooguy]

Originally Posted by dmjlambert

That front pin is the metering valve action. The metering valve delays brake action on the front disc brakes until rear brake drum fluid pressure starts to overcome the return springs. It is something separate from the shuttle valve that we have been talking about. The shuttle valve detects leaks in one of the two brake circuits and operates the warning light. The 3rd valve inside there is the proportioning valve, reducing the pressure going to rear brakes, since drum brakes do not require as much pressure as disc brakes.

so does this mean the prop valve may be working correctly and the shuttle valve is just stuck

[dmjlambert]

I think you would need pressure gauges and some sort of a test bench set up, and know the specs, to know if the proportioning valve works exactly as designed. You have 3 valves there in that there combination valve, from front to back they are metering valve (only affects the front brakes), shuttle valve (which warns you of a leaky circuit, front or rear), and proportioning valve (only affects rear brakes by reducing pressure). If it were mine, I would be interested in trying to get the shuttle valve unstuck and see if it works normally, for which you don't need any special equipment, just a procedure. The other 2 valves I don't think there is a testing procedure in the service manual, but I could be mistaken. Anyway, if one of the valves has an obvious problem I would not suggest to worry much about the other 2 valves until you determine if the shuttle valve can be nudged out of the seemingly stuck position. You haven't indicated if you've tried to do that by opening a bleeder and pressing the brakes while observing the light, or with bleeders closed the stomp method. Some people here on the forum have said they removed the switch and looked down in there and moved the shuttle with a pick. It might take some PB Blaster or something like that to remove the switch on a rusty old valve. Do you happen to have any leaks that you can find? Perhaps the warning light should be on because you have a leak, what do you say? Pull those drums off and look, and look at the front calipers. All the tubing and hoses, brake fluid level, etc. Sorry, all I have is more questions.... :-)

[RichardJ]

>>and the shuttle valve is just stuck<<

AGAIN, it's not necessarily stuck. It will stay where is until YOU move it. You can move it manually with a pick or with pressure by cracking a bleeder valve.

It is good that you saw the Metering Valve pin move OUT. On the cast iron Combination Valves, some move IN and some move OUT. The Metering Valve takes roughly 75psi to open.
When you have a system with air in the front lines and you are trying to bleed it, you must manually pull the pin out and hold it there while you are bleeding.
There are a couple types of tools made to hold the pin out. On the other type of Combination Valve, the pin must be pushed in and held there.

I'm still not hearing anything that says definitively that you actually have a brake problem. The fact that the Metering Valve pin moves as it should, makes me wonder if the brakes even need bleed.
With the engine off, do you have a good hard pedal and does it stay there? Pump the pedal three times and hold it there. You need to vent the vacuum from the booster.

[dooguy]

I have tried to remove the switch but it does not want to move. I was hoping I could move the shuttle to free it from the top.

truck is sitting on asphalt and there is nothing leaking around brakes that I can see.

reading about why brake lite was on sent me down this road. To me the brakes don't seem like they stop like they should

brake pedal is firm

[dmjlambert]

A just thought of another thing, symptom of rear brakes locking up earlier than maybe they should and not feeling right is a leak of the brake cylinder, which you can only tell if you take the drum off and check it out. Brake fluid on the braking surfaces will sometimes make it grab. In some cases a build up of brake dust in the brake drum causes bad brake feel. I think if your brakes don't feel right, you have to check it all out thoroughly.

[RichardJ]

We actually know very little about your truck.

The taller the diameter of the tire, The bigger the brakes needed.
The heavier the spinning weight of the wheel/tire, the bigger the brakes needed.
The heavier the vehicle, the bigger the brakes needed.

Todays trucks are bigger and heavier and have bigger wheels, but they also have much bigger brakes than 1971 trucks if that's the comparison you are making.

[dooguy]

http://67-72chevytrucks.com/vboard/a...1&d=1689382061

Just a stock 71 k20 with stock wheels and tires

[MikeB]

Originally Posted by MikeB

However, earlier this year I replaced the switch because it was simply worn out, causing the light to flicker. Problem solved! But now, after the truck has been sitting for five months during a wiring harness installation and other mods and repairs, the warning light decided to come on and stay on. I'll be taking it out in a few days for it's first drive since February to see what happens.

Well, I drove the truck 7-8 miles and used the brakes aggressively several times. Warning light is still on. Brakes seem to work fine as in the past, but I guess it could be all fronts and no rears.

What's my next step? Guess I could always trying bleeding a rear drum to see if the rear line is "shut down".

Again, combo valve is probably the original from a 71-72 C10. The switch is new and worked properly before truck went into hibernation for 5 months. Disconnecting the tan wire from switch connector causes the dash light to go out, so it's not a short to ground anywhere.

[dmjlambert]

I suggest step 1 is check thoroughly for leaks, including removing the drums and looking, and check reservoir fluid levels. Step 2 see if the light goes out when you disconnect the wire from the combination valve to make sure the combination valve is actually causing the light to be on. Step 3 use the method RichardJ gave earlier about opening a bleeder and press the brake and observe the light. The service manual gives the procedure for checking the light switch in the same way. If the light does not go out between moving from front to back and back to front as you open the appropriate bleeders and press the brake, then it may be your combination valve needs sent out for rebuilding. You may need to use the tool RichardJ shows a picture of to disable the metering valve while you bleed. I have not done that and not had a problem just letting the metering valve work normally during front brake bleeding, so I don't know how important that step may be, I suspect not of great importance. Step 4 (or before step 3 if you want) take the switch off and look and poke around in the valve to see what's going on and perhaps use a pick to move the shuttle and see what you get. I have not done that myself but have seen mentions of people doing that.

Since you have the valve from Inline Tube laying around you can verify using that fixes the problem. If the replacement fixes the problem, send out the original for rebuilding when you can tolerate the spending. If it has been a year or two since flushing the brake system, I recommend mostly empty the master cylinder and fill with fresh brake fluid and bleed all 4 brakes for at least a couple cups of fluid through each and watch the fluid color and cloudiness until you're sure you're getting new brake fluid out of the bleeder. I recommend DOT 3 or DOT 4 and not DOT 5. And if you have DOT 5 thoroughly flush it out of the whole system because DOT 5 does not mix with the other types.

[MikeB]

Originally Posted by dmjlambert

I suggest step 1 is check thoroughly for leaks, including removing the drums and looking, and check reservoir fluid levels. Step 2......

Thanks, man!

[MikeB]

Well, I pulled out the switch, verified it was working, and tried to move the shuttlecock. Didn't think I was making any progress, but reinstalled the switch and snugged it down. No more brake warning light! Haven't taken the truck for a drive, but I've pressed hard on the pedal several times with and w/o the engine running. Still no warning light (and no continuity to ground at the switch).

I also checked the fluid. Levels are fine and it's not too dirty, but there's a little junk in spots on the bottom of the reservoir. I'm embarrassed to say how old my fluid is, so guess I'll be bleeding the brakes soon.

Need to wrap my head around bleeding procedure, because I've never even taken into consideration the combo valve.

One more thing: What are the problems with the aftermarket combo valves? Reliability? Rear brake line pressure not ideal for trucks?

[68 P.O.S.]

Originally Posted by MikeB

One more thing: What are the problems with the aftermarket combo valves? Reliability? Rear brake line pressure not ideal for trucks?

They're not designed for these trucks, so the specs aren't correct and can cause rear brake lockup issues. Supposedly the aftermarket combo valves are modeled after 70s/80's Camaro units, tho I'm not sure on the truth in that. Also, a lot of the brass aftermarket combo valves like to leak from the metering valve section up front.

[dmjlambert]

If you didn't think the shuttle was moving with a pick then it may have too much rust and gunk in there to move freely and may not do its job in the future to warn of a leaky brake circuit. So I recommend follow the service manual procedure to test the warning light switch while you bleed and flush out your old brake fluid. Old brake fluid has a lot of water in it and can make components rust from the inside, but new brake fluid protects. I had a mechanic tell me if I flush the brake fluid every year or two, the brakes will last forever (except for the pads/shoes and rotors/drums that are supposed to wear), and I believe him.

Regarding the new combination valves, I have just become tired of replacing parts with parts from China and especially parts that don't have a lifetime warranty. I have seen quite a few posts here on the forum of people trying to get one of those brass block type of aftermarket combination valves that doesn't leak, especially at the metering valve. And to me they look cheap compared to an original cast iron one that has been freshened up and has a layer of cast iron color paint. The high cost of getting the valve rebuilt once every 50 years or so is OK. And White Post offers a lifetime warranty. So all those factors lead to me getting mine rebuilt.

[68 P.O.S.]

Like dmj above, I'd have an original for the truck rebuilt, period. I wouldn't mess with it anymore. I've had my truck sideways from hard braking due to crappy aftermarket combo valves way more than I like.

[MikeB]

I sent White Post Restorations a message asking for price and turnaround time.

Thanks for the recommendation.

[dooguy]

Is there a procedure for removing prop valve in the manuals?

[dmjlambert]

I don't recall, but there is a link to the service manual on post #28 of this thread. I suggest download it and give it a browse. I think I remember reading if the combination valve is bad it is not rebuildable and should just be replaced. But those are instructions from back in the day when you could actually get a suitable one new.