Parking Brake Adjust

[Schurkey]

The "Knock-Out" is stamped into the backing plate and is designed to be removed, then yes, a rubber plug installed. The knock out is Hinged in the middle, both top and bottom, so go for the far ends with a nice large punch. No need for a cold chisel. Just position the punch on the tab at the end, then hit it. I have removed hundred of these doing brake jobs from the mid 60's going forward. Once they break loose on one end, grab it with side cutters and bend back & forth until it breaks off.

I've seen backing plates that "seem" to have the knock-outs punched into them. In fact, it's as if the stamper didn't punch them deeply enough. No amount of beating on the "Knock-out" would actually knock the thing out.

As other have said, make sure the "Star" wheel adjustor is free, perhaps some anti-sieze, but adjust and turn the drum until it becomes too tight for comfort, then back off. A small thin flat blade screwdriver is used to hold the advancing arm of, then turn the opposite way to back off a little. If your drums have NO outer groove, then they should still come off while rotating with your hands. On the back side of each shoe, there are 3 spots that the shoes backing plate (Under the friction) has a "V" shaped spot on both sides. The backing plate can wear in this area and create a groove so big, the shoes will catch on them. Replacement use to be normal, but a 4 1/2" angel grinder held just right can dress these 6 spots on the backing plate. The pads new have a cross-hatch pattern, but if ground of, still place a drop of high-temp brake lube on this area.

I'd be afraid to merely grind the shoe resting-points flat again. I've heard about MIG-welding the worn area to build it up, then grinding flat. Not something I've needed to do, although I probably should have rather than replace the backing plate.

Some amount of wear on those six resting-points is acceptable. If the brakes work good, and the shoes don't wear excessively, don't obsess.

Calipers can be repaired for $5 per side with a rebuilt kit.

I just priced kits for a pair of calipers from a '94 K1500 extended-cab. About seven dollars. for each caliper. But point well-taken. If the metal of the caliper--specifically the piston--is good, rebuilding the rest of the caliper is cheap.
https://www.napaonline.com/en/p/UBP2005?
But that doesn't get you the caliper guide seals and o-rings. Rebuilt calipers can be a bargain, if they're a popular number. They can be a horrible money-pit if they're intended for a "not popular" application.

But I'm an extreme tightwad. I "rebuild" calipers using the original rubber parts, just cleaned and lubed. (see below) If they work and don't leak, I'm money ahead. If they leak, I've wasted time but very little money, and the calipers get turned in as "cores" on some rebuilts, or I actually buy fresh rubber and pistons or whatever to "properly" rebuild them.

If you have compressed air, place a 1/2 - 3/4" piece of wood or metal plate on the out-board pads side, hit with air, piston comes out. Once the piston is out, if chrome plated, inspect for loose plating or rust areas and replace if needed. The bottom of the caliper is the bottom of the bucket. This is where all of the junk collects, you will be shocked if these are original and wonder how they worked. Brake spray and green scotch-brite will clean the bore nicely.
The lathe-cut or flat seal goes in first, then using brake assembly grease, lube the seal, bore, then slide the piston inward. The outer seal has to be driven into place, just the outer edge. It really helps to have a seal driver. If rebuilt, or new re-mans, either way need the holding frame clean, greased and the caliper sliding with ease.

DO NOT try to "catch" the piston as it pops out of the bore. You'll be going to the hospital with crushed fingers. Wood block, or cram a bunch of shop rags between the piston and the caliper. Rags do a fine job of cushioning the piston when it blows out.

Don't mistake piston/seal assembly lube for the sort of grease you'd use on the caliper sliders. I use BRAKE FLUID to lube the seals and piston. That way there's no possibility of seal-swelling from petroleum products.

On "MOST" calipers, I prefer to have the two seals--the square-cut "fluid" seal, and the bellows "dust/splash seal" both installed in the caliper bore. Then blow some air into the caliper via the brake hose hole to "inflate" the dust seal and slide the piston in place.

I've seen seized pistons where normal shop air at ~130 psi won't budge them. They get removed using a grease zerk threaded into the brake-hose hole, and then use a grease gun's 1000+ psi to pump the piston out. You waste a lot of grease that way...but it will do a fine job of removing a stuck piston. Calipers iwth multiple pistons make this harder--you have to block a non-seized piston so that all the pressure can only act on the one that's stuck.

I just uploaded photos of a caliper "overhaul" as part of a front-suspension rebuild, last night. There's a series of hellishly-long posts in that thread, but the caliper rebuild is this post:
https://www.gmt400.com/threads/front-end-overhaul.45214/#post-1056197

Entire thread here:
https://www.gmt400.com/threads/front-end-overhaul.45214/

The brakes are a system and if one thing is wrong, it will effect the entire system feel and operation. This include the condition of rusty lines, hoses and especially any recent work.

Well said.

There is a residual line pressure device in the master cylinders rear brake port behind the metal insert to seat a double flare / inverted flare, that keeps about 10 PSI to keep the wheel cylinder cups inflated.

GM quit using residual pressure valves in the master cylinder starting in about 1970. If they're using them again, I don't know about it. Far as I know, they "improved" the wheel seals enough that just having the master cylinder mounted high on the firewall provided enough pressure to keep air from entering the wheel cylinder via the rubber seals.

The proportioning valve holds off rear brake pressure as to apply the fronts first, then rears second. It also moves an internal valve the effects you brake switch. It turns the light on with the park brake off. If pressure has changed so much, the light will come on, indicating a loss of pressure.

Sort of.

The COMBINATION VALVE for front disc/rear drum brakes includes a PROPORTIONING VALVE that reduces rear-wheel lock-up, a METERING (holdoff) VALVE that prevents the front brakes from applying until the rears have enough pressure to overcome the return springs, and the "SAFETY SWITCH" that turns on the dashboard warning light if hydraulic pressure from the master cylinder isn't reasonably equal (Hydraulic failure in either the front or the rear hydraulic circuit)

 

[BOWHNT]

Thanks everybody. Lot's of good info.

 

[Gibson]

GM quit using residual pressure valves in the master cylinder starting in about 1970.

When I converted the rear drums over to discs on my '80 K20, there was a residual valve for the rear drums located in the master,, of course there was not a residual for the front discs.
I pulled it out, and replaced the combo valve with a disc/disc unit.
Also added an adjustable valve in the rear line,, the problem with the adjustable valves is you need a different setting every time you go from a loaded vehicle to an empty bed.
Thankfully, in the newer rigs, with a good working ABS, you don't need to add an adjustable valve, you can cram all the pressure you want in the lines and let the ABS do its job.

 

[Schurkey]

GM quit using residual pressure valves in the master cylinder starting in about 1970. If they're using them again, I don't know about it.

When I converted the rear drums over to discs on my '80 K2500, there was a residual valve for the rear drums located in the master,, of course there was not a residual for the front discs.
I pulled it out,

Interesting. I'm a little surprised, but not stunned. I guess I know about it, now.

I wonder when they re-introduced them, and on what vehicles (or perhaps they were across-the-board on anything with drums.)

 

[Gibson]

I wonder when they re-introduced them, and on what vehicles

Yeah,, heaven only knows,, I think that GM was pretty good at "mixing and matching" with a lot of their parts.
Back in those days it was easier to use-up stuff from all kinds of parts bins because their was so much interchangeability.
Edit; I bought the truck in '96, so it was already 16 years old, a previous owner could have changed the master for all I know.