I'm at a loss..

[PrairieDrifter]

Right and unfortunately none of us are there with you. So we throw out our best advice.

Why don't you drive the truck until the brakes are dragging good. Reach under the truck and crack the bleeders for both the rear drums. If it's hydraulic issue there should be a good spirt of fluid and the brakes should free up. If it's mechanical inside the drum then they'll still be dragging and locked up.

I'd just be worried about it giving me a false positive. Since everything is heating up, I'm assuming the brake fluid would heat up in the wheel cylinder and push out fluid regardless. The fact that I adjusted the brakes off, and only lightly touched the brakes twice then drove ten miles of highway, and they still got hot. Tells me it's not a hydraulic or input problem.

Again not saying anything isn't possible, but i don't want to throw a third set of brakes on it just in hope that it'll work. If I can't get quality replacement parts from parts store then what's the point, thats one of the reasons I love squares. I can go anywhere in America and I'll have parts availability. I'd rather not go aftermarket but, by the time i do another set of brakes(would be the third), i would probably be 3/4 of the money into it, as a disc conversion. Now that I've done a master and am considering doing the booster.

As well as I've heard nothing but bad things about the replacement proportioning valves. They leak right away or within 3-5 years, and who knows in how many miles. But I daily this truck, in ND, it probably won't last for me with my luck.

 

[SirRobyn0]

I'd just be worried about it giving me a false positive. Since everything is heating up, I'm assuming the brake fluid would heat up in the wheel cylinder and push out fluid regardless. The fact that I adjusted the brakes off, and only lightly touched the brakes twice then drove ten miles of highway, and they still got hot. Tells me it's not a hydraulic or input problem.

Again not saying anything isn't possible, but i don't want to throw a third set of brakes on it just in hope that it'll work. If I can't get quality replacement parts from parts store then what's the point, thats one of the reasons I love squares. I can go anywhere in America and I'll have parts availability. I'd rather not go aftermarket but, by the time i do another set of brakes(would be the third), i would probably be 3/4 of the money into it, as a disc conversion. Now that I've done a master and am considering doing the booster.

As well as I've heard nothing but bad things about the replacement proportioning valves. They leak right away or within 3-5 years, and who knows in how many miles. But I daily this truck, in ND, it probably won't last for me with my luck.

I understand your concern, but as long as everything is functioning properly up stream it is impossible for any heat related fluid expansion to cause pressure. If that were possible every time you hauled a load or a trailer and went down a long hill and heated them up a bit you'd end up with stuck brakes. Also that's a time tested method used at all the shops I've ever worked at to figure out if it's a mechanical problem or hydraulic issue. In theory if you wanted to you could pump fluid backwards though the lines and master cylinder, assuming there is no restriction up stream.

I have seen vehicles where you'd let them sit overnight, pump the brakes a couple times and the rear wheels would be dragging. Open the bleeder you'd get a squirt of fluid and they'd turn again, because the rubber hose was literally acting like a one way check valve. So you could do that, since it only took two pumps before. Fire up the engine pump the brakes 5 times really good and hard and pop open the bleeder on the worse of the rear wheels. If you get a squirt of fluid you know it's hydraulic, if you don't then you have proved is in mechanical. If you do get a squirt, you could pump them again and crack the line just before the rubber line to see if you got a defective line and work your way up to find the issue. That's how we'd handle it at the shop, if it'll start dragging after a few good pumps.

I daily my truck and use it for the farm. I get it. It needs to be reliable. It sucked when it was out last year for 3 weeks whilst I had the transmission rebuilt, but at least I got warning signs and could schedule it. For that reason I would never ever consider an aftermarket disk brake conversion because speed of parts availability. I just put 200 miles on it today hauling feed, I really don't wanna have trouble with a load of feed on board and 100 miles from the farm! I get where your coming from.

 

[PrairieDrifter]

I understand your concern, but as long as everything is functioning properly up stream it is impossible for any heat related fluid expansion to cause pressure. If that were possible every time you hauled a load or a trailer and went down a long hill and heated them up a bit you'd end up with stuck brakes.

This makes sense. But the heat generated from towing isnt even close to how hot my drums are getting imo. Maybe with some emergency braking or going down a mountain.

I did crack a bleeder after my 30 mile drive and mashing on the brake pedal before I did it. Nothin. The babiest pressure release, what you'd expect from a sealed pressurized system. Barely did anything more than a gravity bleed. Which it then proceeded to gravity bleed just fine.

So it seems to be mechanical. It's gotta be drums and shoes. It also correlates with not feeling at all like a hydraulic drag. I've dealt with that before, that always has very similar symptoms, like you said, the pressure increases every time you hit the pedal. Now it may not act 100% the same way with all, but they all act similar.

This thing just drags just because. I bet I could adjust the brakes almost all the way off, pull onto the highway without touching the brakes AT ALL and they would still drag like they are.

 

[mxer147]

Brakes+ an option?

 

[Grit dog]

This is quite perplexing. And I can say that a good large % of the folks on this forum and responders to this thread are some very capable backyard or professional mechanics. I’m stumped and I’ve been around enough drum brakes. @SirRobyn0 had the last best suggestion which you also said isn’t the case.
I’ve never “hated” drum brakes but I loved the simplicity of disks and wuz happy when rear discs became the norm (although somehow I now have 3 vehicles with rear drums and 1 with 4 wheel drums, all that work wonderfully).
This would push me over the edge though and there’d be some disc brakes goin on that ****!

 

[Grit dog]

M o f o is a bad word, lol.

 

[PrairieDrifter]

This is quite perplexing. And I can say that a good large % of the folks on this forum and responders to this thread are some very capable backyard or professional mechanics. I’m stumped and I’ve been around enough drum brakes. @SirRobyn0 had the last best suggestion which you also said isn’t the case.
I’ve never “hated” drum brakes but I loved the simplicity of disks and wuz happy when rear discs became the norm (although somehow I now have 3 vehicles with rear drums and 1 with 4 wheel drums, all that work wonderfully).
This would push me over the edge though and there’d be some disc brakes goin on that ****!

Agreed on all points. The only problem with drums ive ever had were with my 06 Colorado. It ate rear shoes but didn't drag. It was just a ****** hardware design.

You could most definitely say i am over the edge lololol

 

[PrairieDrifter]

M o f o is a bad word, lol.

Of course. How could we be so stupid lol

 

[Grit dog]

Well I’m as interested as anyone if you figure it out because seems a total head scratcher to me. There’s only so many parts involved and it’s not like drums were cutting edge tech in the 70s….

 

[andybflo]

I'd bet money it's the proportioning valve or misadjusted master piston. The shuttle valve in the PV2 (Disc/Drum, GM) can absolutely stick and cause residual pressure. Infrequently. Randomly.

All you need is old brake fluid with a high particulate/moisture content to contaminate it once. My '86 C10 must've gone through that period of apathy at one point. I'd get a flash of the "brake" light very, very, infrequently, sometimes a squishy pedal, or a perceived drag. I owned a PV2, stuck it in, and boom.. Never another issue. Disassembled the OE one, and the seals/piston were gummed to heck.

And, yes, it also meters and times fluid. I'll be happy to provide you a GM whitepaper on it's design/method of action, if you'd like. It's not just a "if you run one circuit dry" switch; it times, and releases the front brakes I believe ~50ms later to coordinate with drum engagement, so your car doesn't have rear lockup before disc engagement. That's spring actuated, and even holds a very slight residual pressure in the rear line.

If it fails, or is gummy, it can cause a multitude of problems. They're $50, and your part is four decades old, with maintenance you can't verify for the majority of it. I've restored about 50 GM disc/drum cars and trucks, and replaced about a half dozen as "bad" for people (it's a side hustle/hobby.) They're all old and crusty at this point. Consider it suspect, and buy a "pin" tool before you bleed it for $5 on Amazon. If you don't know what that is, I'll happily point you in that direction.

I've had a "one-way-check-valve" hose happen on an old Mopar, it does happen. Rare, but it was an internal failure, and my buddy (who's a very competent mechanic) brought it over scratching his head. Brake straight up locked up on the NYS Thruway. He limped it to the side of the road, and had it towed home. It would release, sometimes, and drive fine, then... Bam. I suggested trying it, based on an old training video, from when I sold parts in college. After a lot of arguments and $25, he was back on the road, and drove the winter beater for another few years.

Right now, other than swapping/adjusting parts you've already swapped/adjusted before with the same results, what have you tried? How can/do you plan to block and tackle each system?

I'd start with a pressure gauge, four jackstands (or a lift), and letting the truck run in "D", elevated for fifteen minutes. Have someone jump in truck, squeeze brake pedal, and look at residual pressure at various points. It's gonna be tedious, but it'll tell you if hydraulics are the issue.

If not, what you're doing is asking for advice, and saying "nah, can't be that", and defaulting to the same thing you've tried several times... With no added benefit until now. If you're convinced it's shoes/drums/HW, replace them all. Mic all of them before install.

 

[MikeB]

I chased a similar problem with the rear brakes on my 82 C10. No drag at all when cold, but one side would hardly rotate when hot. Using an IR temp gun, I recall one side being 100+ degrees hotter than the other. I swapped the drums from side to side and the problem followed the suspect drum. Drum diameter was fine when cold, but it must have been getting out of round when it heated up from normal use. It then got more and more out of round when I continued to use the brakes. It's possible it was bad from the factory, but I did have the parking brake adjusted fairly tight, which could have caused it. Anyway, a new brake drum solved the problem.

 

[Lowered87]

If you are okay with spending a few dollars, Wilwood carries a Combination Proportioning Valve and Residual Pressure Valves that might solve your problem. Put the residual pressure valve on the line to the front discs to increase the speed at which they function. The combination proportioning valve has a knob that allows you to control the rate at which pressure is applied front and back. Wilwood has a lot of resources on there website to help you select the correct model. Summit racing is a place to purchase from although Wilwood is sold through a lot of different vendors. You are probably looking at less than $200 in parts and the valve will fit in the factory Chevy location. The residual pressure valve goes dirrectly on to the master cylinder. The purpose of the adjustable proportioning valve is to allow use of factory brake components and then replace rear drums with discs later. This information comes from Wilwood as I am still rebuilding my vehicle.

On another note, I had a collapsed inner hose acting like a check valve on my Ford F-350 work truck. Hoses were perfect on the outside so I changed them both and problem solved.

 

[Lowered87]

DS715 is the proportioning valve. Little over $100.

 

[84C20HD]

I may have missed something here as this is a very lengthy thread. Are these drums NEW, if not, how many times have they been CUT? OLD drums with ridges can also force new shoes into the backing plate and or the drum. Brake fluid and rear end gear lube even in the smallest amount will cause a shoe to swell , expand and never return to normal size. If these are old drums see if they are beyond condemned SIZE. An over extended cut drum can cause brake parts, particularly upper brake bar between the shoes to get extended beyond it's intended length, and cause that bar to make the brakes stay extended , which will cause drag. Ever try a hard brake test when cold to see if all wheels are locking up? Try a hard skid. Maybe you only have rear brakes?

 

[MikeB]

I would not assume this is a combination valve problem. And I would certainly would not buy an aftermarket brass valve (like the common PV2) that is prone to leaking. (Edit: Wilwood makes good stuff, so they would probably be the exception.)

I haven't read all the posts here, so I don't know if the brake warning lamp is on or flickering. And if it is in one of those states (especially flickering), it could be the switch is worn out. I just replaced the switch in a 71-72 combo valve that was mechanically worn out and causing the brake light to flicker. The pin (on the left side) had lots of "wobble" in it. There was nothing wrong with the combo valve though.

Have to say that the proportioning valve, residual valve, and warning switch are all part of the combination valve. And at least one of you needs to google "front disc residual valve" to learn how that part operates.

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