Dragging Rear Brakes. Pressure Reg or Metering Valve?

[BlackoutSteve]

Hey guys, I am having the problem of dragging rear drum brakes that get progessively worse in stop go traffic. The associated symptom is that the brake pedal does not fully return. Lifting it with my foot often releases the dragging rear brakes.
After have them recently lock on and having no choice but to drive the short distance home, naturally they killed themselves with heat and I rebuilt them with new everything except 3000 mile old wheel cylinders which I checked and found they both moved freely in their bores.
I have also found that they drag heavily after they are warmed up and parking the car for around 10 mins allows them to release, so now I'm suspicious of heat somehow..
I even welded the backing plates' rubbing blocks and sanded them back to eliminate the grooves worn in them over time. I know the rear drum assemblies themselves are good. It would be an enormous coincidence for both rears to develop the same problem at exactly the same time.
This issue seems to have been present in the system since the car's recent resto debut, and since then I have...

Made sure their is ~1/16" clearance between the booster's pushrod and the master's piston,
Bled the entire system,
Replaced the EIS replacement master with the correct oe type (repro of the 5468165) from HBC. (Bench bled, yes.)
Installed a vacuum pump and tank that maintains ~20in/hg vacuum, (as the roller managed only ~8")
Swapped the brake pressure regulator valve with another new one.. This is the valve that is mounted on the subframe below the driver's seat.

The system also has a metering valve which was rebuilt and seems to work.
Do either of these two valves maintain a small amount of residual pressure in the line for the use with drums or is that the master's job? I know front disc / rear drums require it and the factory did it somehow.

Because most of the likely causes I thought have been replaced or eliminated with new parts, I'm wondering if anyone has had problems with these valves maintaining pressure in the rear line causing the rear brakes not to release fully.

The other component I am suspicious of is the 8" dual diaphram booster. It and the pedal mechanism have been checked for binding but they move freely.

Help!
It's driving me mental..

[BlackoutSteve]

One thing I have just found out (but don't he it necessarily being a contributing problem in my case) is the boneheads who make the repro pressure regulator valves, bore and hone for the plunger and then they punch the bracket on. ..in that order..
The bore on my valve was 0.020" out of round and the spring was binding with in it. I had to pull the spring out using pliers and force it back in.
I rebored it to 0.50" and now it drops in as intended and moves freely.

[tom]

Are the adjusters working correctly ?

[Steve68]

The associated symptom is that the brake pedal does not fully return. Lifting it with my foot often releases the dragging rear brakes[/quote  Have you fixed this?

Do this.  Rear off ground, rotate wheel by hand (should rotate freely), press emergency brake pedal down two clicks.  Does this lock wheel?

Steve

[JohnZ]

Replace the rubber flex line to the rear axle - they deteriorate from the inside out, and act like a check valve, causing exactly the symptom you describe.

Neither the metering/hold-off valve nor the proportioning/pressure regulator valve have any residual pressure function - that's handled by the RPV in the rear (drum) outlet of the master cylinder.

[Sauron327]

Here's that commonly overlooked flex hose problem. You'll also want to flush all that boiled fluid out of the system.

[BlackoutSteve]

Adjusters are working fine. When cool, the drums slide off no problem. Also, if the brakes locked or dragged mechanically, no amount of time would "relax" them.

Once the system is bled, the pedal seems to return nicely for a short while, then it starts to stick more as if there is air in the rear line. (I am beginning to have suspicions of the pressure regulator valve introducing air passed the plunger seal and into the rear line. I might post in another thread that discusses this part.)
I'll check the park brake adjustment, but fairly sure it's not the cause of the dragging. When the brakes are dragging, there is very light resistance in the park brake pedal until several clicks toward the bottom of it's travel like normal.

I have an new replacement flex hose as I am aware flex hoses acting like check-valves like you say. The reason why I haven't swapped them yet is because the hose on the car is brand new and only ~3000 miles old.

Old fluid is out when the rear drums were rebuilt and refiited..

Thanks guys

[BlackoutSteve]

I couldn't find the thread on the rebuilding of the pressure regulator valve (I don't think there is one yet) but I have found this by Scott Hollenbeck (username here is sah62)
 https://www.musclecarresearch.com/kh-multi-prop-valve-autopsy

First of all, the repro valves don't have Tru-arc retainers, the thin washer, or the seal found in front of the said retainer on the lower stem of the piston.

My question is this.
Is the upper ring-seal on the plunger/piston supposed to prevent brake fluid passing into the lower part of the valve where the spring is located?
On the repro I have, the knobs who made it also fitted the wrong size lower o-ring and that caused fluid to pass out of the lower nut. I replaced the o-ring with the correct size and now all is fine regarding leaks.
So, if the upper ring-seal allows fluid to pass into the lower part of the valve, how can this valve ever work when an incompressable fluid is on both sides of the piston/plunger seal? -because with the lower o-ring leak that I had, it's quite clear the upper ring-seal is not a good fluid seal..

I also fear that this air cavity that originally exists in the lower portion of the valve when fitted is getting passed the upper ring-seal and into the rear system during use.

[Steve68]

The parking brake should stop the wheels from tuning long before the pedal reaches near the bottom of its travel.  With the parking brake pushed down two clicks the rear wheels should not turn by hand.  If either wheel does they need an adjustment.

[Sauron327]

Here's another thread on the proportioning valve: http://www.camaros.org/forum/index.php?topic=5952.0
Not sure why you think air being introduced into a hydraulic system will cause a pressure build. Air will cause less pressure. If you feel the p. valve is causing the problem, although it should not be, just install a bypass to test your theory. I don't put 100% trust in new part quality, your new flex hose could still be bad.

[BlackoutSteve]

Thanks Steve, I'll check it out..

Here's another thread on the proportioning valve: http://www.camaros.org/forum/index.php?topic=5952.0
Not sure why you think air being introduced into a hydraulic system will cause a pressure build. Air will cause less pressure. If you feel the p. valve is causing the problem, although it should not be, just install a bypass to test your theory. I don't put 100% trust in new part quality, your new flex hose could still be bad.

I am not sure if there is a pressure build up, but one of the causes of the brake pedal not returning is air in the lines. That valve can easily be introducing air if the upper ring-seal leaks.
I do intend on bypassing the valve. I will simply remove the guts of the valve and re-bleed..

[NoYenko]

I am not sure if there is a pressure build up, but one of the causes of the brake pedal not returning is air in the lines. That valve can easily be introducing air if the upper ring-seal leaks.

Steve when ever I had air in the brake system the pedal allways returned to the top. The pedal returns to the top by the spring pressure in master & booster. After the brakes are dragging I would open the bleeder on the master without any pedal pressure and see if you have any pressure at that point. You should not have any pressure. Next with engine off, disconnect the vacuum to the booster and see if the pedal comes up, if it does you have a booster problem. George

[sah62]

I couldn't find the thread on the rebuilding of the pressure regulator valve (I don't think there is one yet) but I have found this by Scott Hollenbeck (username here is sah62)
 https://www.musclecarresearch.com/kh-multi-prop-valve-autopsy

I know of one Wagner proportioning valve used on Fords that can cause the rear brakes to drag if the internal piston doesn't move freely. I've never heard of that happening with this K-H valve, but I imagine it's possible. As noted above, removing the internal piston to bypass the proportioning function will confirm if the valve is the problem or not.

[BlackoutSteve]

Thanks Scott. 

I have narrowed it down somewhat, and have found that the issue is most likely the line-lock that is installed on the subframe and is between the distribution block and the pressure regulator valve.
(Yes, I have mounted the line-lock on the rear system so it isolates the rears from working which gives me normal use of the fronts during a burnout.)
With the car warmed and the rear brakes holding on, cracking the flare-nut on the distribution block under the master cylinder, no fluid was expelled.
But when I cracked the flare nut on the inlet side of the pressure regulator valve, fluid was expelled and the rear brakes released immediately. The line-lock is in between.
This also proved that the flex line, wheel cylinders, park brake adjustment and rear brake assemblies are all OK.

It's a Moroso (44050) line lock and their manual says they are sensitive to heat. They aren't that close to the headers on my car, but the hot air flow from the headers would be warming the line-lock somewhat.
To see if that was the case, I made a small heatshield over the line-lock and it didn't eliminate my problem, but it did reduce it, so I think I'm on the right track. (BTW. The line-lock is creating the fluid restriction all while switched off.)
I will now consider options regarding the replacing, rebuiding or relocating the line-lock.

Thanks a heap to those who have helped me in this thread.

[Mike S]

Ohhhhhhh- a line lock....burnouts....lots of smoke.......next comes the high revving clutch dropping holeshot.........I have many a fond memories of those days 

Mike