Proportioning valve problem?

[MikeB]

I installed some new brake hoses on the truck and decided to completely replace the brake fluid. When bleeding the rear brakes (both sides), the pedal would barely move. It was almost like the bleeder valves weren't open. However, they were open and a little brake fluid was coming through.

Moved to the front and both sides bleed just fine, with the pedal going all the way down to the mechanical stop.

Before I started the job, I replaced the switch with one of these to make sure the pressure differential valve didn't move.


Then when I had the problem, I removed it and re-installed the switch. The switch is closed causing the dash warning light to turn on. Stepping on the brake pedal doesn't help. So, it seems like the valve is stuck in the rear position. But the light had never come on solid before. It had flickered somewhat, but replacing a worn out switch cured that. ***See post #6 below to see what I think was causing the switch to close.

Have to say that for some time now I've felt the truck's front brakes were doing most of the work (more than normal). And when I pulled the rear drums, there was only a small amount of brake dust and the old shoes looked almost new.

The prop valve is from a 71-72 truck and may be the original. Could that be my problem, or might it be the master cylinder?

I have a brand new aftermarket prop valve, but have been reluctant to use it, given all I've heard about leaky seals.

[Sheepdip]

You say "Some" new brake hoses, howz about the rear center hose that drops to your hard lines on your differential? That's a pretty common hang up point when it starts coming apart inside.

[Rust_never_sleeps]

Rears fought me the whole way. Some things that do help:
Good condition rear hose and wheel cylinders
Properly adjusted rears (if they have too much travel, they do something mysterious and block the flow)
Properly centered and pinned shuttle valve(diff pressure valve) in the combination valve. Verify this visually. Re-center it by cracking a front bleeder or pulling it back with a pick, not by slamming on the brakes.

I got no love using vacuum bleeders, but had good luck with the Motive tool. IDHAFC why one worked and not the other

[Dashman]

I had similar issues with poor braking with the original Bendix 0411 combination valve. Thinking that it was rear brakes, I started by replacing them. It didn't solve the issue, and when I removed the drums the shoes were as if they hadn't been used at all. I sent the proportioning valve to a company mentioned here, and it had a gross leak from the metering valve because they had installed the seal incorrectly. They did say that the spool was seized, and they bored and sleeved it. If I had to do over again, the metering valve isn't very difficult to remove, I would remove everything, inspect, and mic' the bore and spool myself. I simply got stubborn to keep an original component, and I didn't feel like sourcing the seals. I would soak it in muriatic acid for short intervals until it frees itself with minimal force. If it's badly pitted or scratched, then I would decide to keep it and send it to be bored and sleeved, or replace it with a brass combination valve.

[MikeB]

My son had a bunch of work done on the truck 6 years ago when he had it. I replaced the rear hose 20+ years ago, but it looks like new, so it may only be 5-6 years old.

It would be a real PITA to replace because the shop welded in a muffler near the frame where the hose is attached to the hard line. But I will have to check it out. I have a couple ideas in mind.

[MikeB]

***Here's an update. I think the pressure differential valve is actually centered. Turns out the switch I bought 1-2 years ago has a hair trigger and moves only .030" or so before it closes. So, even on my new aftermarket prop valve, as soon as I install the switch, it closes. The switch that came with the new valve moves more like a .100" before it closes.

However, that doesn't solve my problem. Now I think I may have a blockage somewhere instead of a stuck pressure differential valve. It is this valve that blocks fluid flow, right?

[Dashman]

Yes, the pressure differential valve will sense a fluid leak, and close the front or rear of the system where the leak is.

I also thought that I may have a blockage in the rear brake line, but when I had the master cylinder and combination valve removed I sprayed brake cleaner and compressed air through both front and rear lines. There was nothing to suggest that there was a blockage.

I had replaced the rear cylinders and brake lines, so it was the rear main line that I opened before the tee.

What about your rear cylinders? Did they sit for a while? Maybe they are seized? Maybe there was a leak, the fluid sucked in some moisture, and they rusted a bit?

[MikeB]

Originally Posted by Dashman

Yes, the pressure differential valve will sense a fluid leak, and close the front or rear of the system where the leak is.

Thanks for the confirmation.

Originally Posted by Dashman

What about your rear cylinders? Did they sit for a while? Maybe they are seized? Maybe there was a leak, the fluid sucked in some moisture, and they rusted a bit?

They look like new. And the brake fluid wasn't too discolored. I'd be surprised if both wheel cylinders are the problem. Looking at this, I think the guys who worked on the truck for my son must have totally rebuilt the brakes, except for the drums. Doesn't look like the shoes were doing much! I'll have someone put a little pressure on the brake pedal and see if the wheel cylinders do their thing.

I think I'll also try the brake cleaner blast from the prop valve output line to the rear tee.

[PbFut]

Do not put anything but brake fluid. That includes air. Compressed air has lots of condensate in it. Brake fluid is very good at mixing with water and other fluids. Doing so drasticly lowers the boiling point of the brake fluid.
As for your bleeding and rear circuit problems. If the lock out plunger is centered and you are sure the tool is holding in place, look to the proportioning part of the unit. It is the short section at rear of the assembly that the rear brake circuit attaches to. That part can fail. I got a brand new one DOA. Took me two days to conclude the new out if the box part was bad.

[MikeB]

Originally Posted by PbFut

If the lock out plunger is centered and you are sure the tool is holding in place, look to the proportioning part of the unit. It is the short section at rear of the assembly that the rear brake circuit attaches to. That part can fail. I got a brand new one DOA. Took me two days to conclude the new out if the box part was bad.

Thanks for the idea. That's one thing on the truck I've never taken apart. After 50+ years, the insides could have a some wear or corrosion.

[Steeveedee]

DO NOT have someone put "a little pressure" on the brake pedal with the drum(s) off. One (or both) wheel cylinders will pop and leak, guaranteed. Then they will need to be rebuilt.

[MikeB]

Originally Posted by Steeveedee

DO NOT have someone put "a little pressure" on the brake pedal with the drum(s) off. One (or both) wheel cylinders will pop and leak, guaranteed. Then they will need to be rebuilt.

That's probably good advice, although it happened accidentally when I was a kid w/o too much drama.

I just now remembered that yesterday when my buddy stepped on the pedal with the bleeder valves closed, the drums would no longer turn by hand. So that means the wheel cylinders are working at least somewhat, but there's still a pressure problem based on minimal shoe wear and how slowly fluid flows out of the bleeder valves.

[RichardJ]

>>based on minimal shoe wear<<

Shoe wear on the rear brakes isn't much of a indicator. Front brakes on front engine, rear drive do 60-70% of the braking. Front brakes on front wheel drive vehicles do 80-90% of the braking. You can replace front brakes on FWD 4-5 times before rear brake shoes need attention.

>>how slowly fluid flows out of the bleeder valves<<

I didn't notice what bleeding method. If vacuum, make sure you are not sucking air past the threads on the bleeder valves.

[Rust_never_sleeps]

typically a failing hose will allow fluid through, but not back, so if yours looks OK, maybe just leave it alone since that's not the behavior you're seeing

[PbFut]

Another idea. When I first got my truck, I flushed the system. The bleeders on the rear were so full of dirt and rust they would not bleed. Removed and cleaned. All was good.

[MikeB]

Just heard back from Old Auto Restorations. Their price to rebuild my original Bendix valve is $240! I was guessing it would be $100-$125.

Thanks for all the advice. Gotta a lot going on now, so will report back later.

[dmjlambert]

I suggest check with White Post restorations, that is where I sent mine. Getting it rebuilt may be a good idea on an old valve even if it doesn't solve the problem, but this thing you said concerns me a little "It would be a real PITA to replace because the shop welded in a muffler near the frame where the hose is attached to the hard line." Rubber brake line close to exhaust and getting the heat may be failing, so the rubber line that you haven't replaced yet would be my first suspect.

I suggest do the switch test as in the 72 service manual here.


In my experience I did not have to disable the metering valve as they mention to do brake bleeding nor to do this test, and to bleed also did not need to use the so-called brake bleeder tool that replaces the switch and holds the pressure differential valve in the middle position. That tool is not mentioned in the manual, it is just a tool sold by aftermarket sellers and not needed, it is fiction.

The pressure differential switch does not stop flow to the front if front is leaking, nor stop flow to the rear if the rear is leaking. It simply lets you know you have a leak and should fix your brakes soon. Also it is not necessary to apply heavy pressure to the brake pedal for the test. I have observed when pressing the pedal with normal pressure that you would use for stopping, if a front bleeder is open to cause a leak, the light will come on. And if you close the front bleeder and open a rear bleeder and press the pedal, the light will go off momentarily and come back on. Close all bleeders and press the brake pedal with moderate pressure once or twice and the switch will reset and light will go off. I made these observations with a 71-72 valve recently rebuilt by White Post Restorations.

[howzzzit]

https://mpbrakes.com/how-to-series-w...ination-valve/

[MikeB]

Originally Posted by dmjlambert

I suggest check with White Post restorations, that is where I sent mine. Getting it rebuilt may be a good idea on an old valve even if it doesn't solve the problem.

Rubber brake line close to exhaust and getting the heat may be failing, so the rubber line that you haven't replaced yet would be my first suspect.

In my experience I did not have to disable the metering valve as they mention to do brake bleeding nor to do this test, and to bleed also did not need to use the so-called brake bleeder tool that replaces the switch and holds the pressure differential valve in the middle position.

I will contact White Post, because I like the idea of having an OEM spare.

The rear brake hose is not close to the muffler. The muffler is close to the frame which prevents getting inside the frame with a wrench. And again, the hose looks like new, as do the rear shoes and wheel cylinders. One of the shops my son used must have done the work.

I had the same experience as you (no problems) with bleeding brakes in the past without holding the pressure diff valve in the center -- on my 69 at least twice, and on an 82 C10. Ignorance is bliss!

Final Update: The new valve is installed and the rears bled just fine. The pedal feels a little different, but in a good way. It has a little more progressive engagement, whereas before it was more sensitive.

[PbFut]

Mike, Good to see you got it working. Did you figure out the actual fail point on the valve you replaced?

[MikeB]

Originally Posted by PbFut

Mike, Good to see you got it working. Did you figure out the actual fail point on the valve you replaced?

No. But I did pull out the valve and spring, and another piece fell out. The only rust I see is on the threads and o-ring. There's a sleeve inside the casting that also looks OK, but not great.

The aftermarket brass valve is working OK and not leaking (yet). It came with a later model switch and pigtail that I'll use. I'm pretty disappointed that the new OE switch replacement failed so soon.

I have to make a confession here. I'm actually wondering if there was any problem at all, other that the switch being too sensitive. When I started to bleed the rear brakes yesterday, I initially had the same problem -- low flow out of the bleeder screw again. Oh, no!

But here was the problem: It's really hard to get much of a turn on a box end wrench with the bleeder's location in the wheel cylinder casting. So I was able to insert the wrench in a way to get maybe another 45 degrees on the turn, and lo and behold, fluid streamed out! Really strange because I have bled brakes dozens of times over the years w/o screwing up like this.

Oh, well. I can tell myself 1) I learned something, 2) I have new fluid end-to-end, and 3) I'll end up with a rebuilt OEM combination valve!

It was also a good day because we fixed a couple brake problems with my buddy's new-to-him 99 Silverado 1500. One of the rear caliper pistons was stuck (!) and the parking brake adjustment was too tight. No bleeding problems, though.