I'm having some odd brake moments and would like some info on the brass junction block in my 72. First, the car was a factory manual brake car. I doubt this makes a difference. Do we have proportioning valves, metering valves (delays rears), combination valve (the above 2 functions in one valve) or no valving whatsoever?

Issue 1: within about 1 block of leaving my driveway I can hear an odd "grrrrr" for about 2-3 seconds when I apply the brakes and then a firm little "clunk" which I can feel through the brake pedal. It does this only once and only after rolling for about a block. Backing out of the driveway and using hitting the brakes does not cause this. The actual braking effect does not change while its making this sound....it does not grab, it does not have reduced braking....it all seems normal. I would think it was some vac operated deal accept for the "thunk" I can feel through the pedal.

Issue 2: the brakes have started to not release under certain circumstances. Its typically when pulling into my driveway, so going slow, making a sharp turn and heading slightly up hill. It happened tonight and it was like driving with the e-brake on getting into the driveway. The brakes did release a little while later.

Now the master cylinder does have a proper clearance between the booster and the piston. The brakes are not dragging. The brake lockup may be at the rear axle. I say this because the cars ass end rose up as I was fighting the brakes to get it up the driveway. The pedal felt normal the entire time. It felt if had fully returned like normal. Pumping it didn't "release" the brakes.

I can see a prop valve or metering valve causing the rear brakes to lock up but not make that odd noise. Although the "thunk" could be the valve unsticking. So, its the valve or I have a bad brake booster. If the booster was staying "on" after releasing the brakes I would think you would be waiting for the pedal to come back up....which is not the case. I converted to power brakes a couple months ago. Unless I have a caliper not retracting.....

Thanks,
Dave

I'd be looking for a problem with the parking brake, from the description. Calipers are fixed at all 4 corners, so no issue with one or more of them sticking on slides. Very rare for a caliper piston to hang like that, either. The p-valve wouldn't cause something like that, without effecting brake operation in some way.
Another thought is that it could possibly be a caliper hitting the wheel. If the clearance is minimal, the wheel flexing on turns can cause it to rub/grind on the caliper slightly, and cause it to feel like it's locking up for a split second or so, depending on how bad it is. I've seen that on aftermarket rims plenty of times.
Just a few ideas to consider....

Another possibility could be the rear differential needing lube change and adding the additive for the clutch packs. When I first got my 77 if I went for a drive it started out ok but after a few miles I could feel the rear drag as you describe. had the rear serviced drove around in circles like suggested and it was ok

1970-73 used a distribution Block

Its separate chambers are fed from both the front and rear master cylinder outlets, with a spring-centered spool located in a channel between the two chambers. If there is a fluid leak or air in either the front or rear brake system, on brake application that difference in pressure will shift the spool one way or the other, and the center of the spool will contact the tip of the warning switch; that grounds the switch circuit, which lights the "Brake" warning lamp in the instrument cluster (the same lamp is also operated by the parking brake when it's engaged). With no air or leaks in either system, the spool remains centered in the block due to equal fluid pressure at both ends of the channel.

1974-77 used Proportioning Valve (combination Valve)

The proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached. This is done to prevent rear wheel lockup on cars with light rear wheel loads.

The valve is designed to have a by-pass feature which assures full system pressure to the rear brakes in the event of a front brake system failure, full front pressure is retained in the event of rear failure.

The pressure differential warning switch is designed to constantly compare front and rear brake pressure from the master cylinder and energize the warning light on the dash in the event of front or rear system failure. The valve and switch are so designed that the switch will latch in the “warning” position once a failure has occurred. The only way the light can be turned off is to repair the failure and apply a pedal force to develop up to 450 PSI (3100kPa) line pressure

|UPDATED|6/26/2014 11:23:35 AM (AZT)|/UPDATED|

I have AZ replacement OE style wheels. There is not metallic rubbing sound but the p-brake is an interesting though. Although why it would "come on" on its own is beyond me. I did adjust it a few months pretty much as far as I could go using the threaded rod on the "horse collar" in the center of the car. You still pull the handle up pretty far before it engages.
I think the main clue here is that odd "GRRRRR" noise I get, one time, about a block out after starting to drive...and then the "thunk" you can a) feel as a thump on the brake pedal and b) hear. You can't feel the p-brake in the hydraulics. A prop valve or metering valve you would. The power booster for sure. It's hard to assess since by the time I run a floor jack under the axle the brakes have release.

  Sounds to me like the E-brake is unhooked, and you have parts scrubbing on the inside. Best take the hat off and take a look-see. Shoes may also have delaminated and are flooting inside the hat.

See my new post under Driveline - failed inner stub shaft bearing. The rotor was getting cocked and dragging on the caliper.