Rear brakes locking up to fast

[Lee H]

Well guys, I'm having an issue with my braking system. 1972 power disc/drum. The rears are locking up rather quickly. Seems little to no front braking action. When I freshened up the truck last year I replaced "all" braking components, hard lines, rubber hoses, all parking brake components, drums, rotors, wheel cylinders, calipers, booster, master, proportioning valve. I originally thought it was a mismatch between the master, booster. I tried an adjustable push rod and no change. I purchased a master, booster, proportioning valve kit from CPP, no change in rear lock up. The pedal travel is right on spec with the service manual and is nice and firm. I loosened up the parking brake cable with no change in rear lock up. No sign of brake fluid or gear oil. There are no crimps or kinks in the hard lines. Don't know if this is recent occurrence but I suspect it has been around for a long time and a recent aggressive braking action brought it to my attention. The rear shoes are nearly worn out after just 8K miles.

So, here's my next steps:
Replace the rear shoes and hardware so that is up to snuff. Trying to find a good detailed picture of a correctly assembled drum setup. The pictures in my service manual are not all that clear. Maybe I do not have all the components installed correctly.

Next will be replacement of the calipers & hoses. All summer I thought the lack of brake dust on the wheels was due to good pads but now I think that is just a sign of the real problem.

Advice is appreciated.

[Longhorn 70]

I have a similar problem. Mine even did it with drum/drum. Watching.

[brad_man_72]

Id start by bench bleeding the front line on the master cylinder, then bleed the calipers.

[Jack_71C10]

I had the same problem. It turned out to be brake fluid on the shoes.

I bleed gallons of fluid through it with no change. Then I remembered that
I replaced a leaking wheel cylinder during my suspension drop. Both brake fluid and axle seal leakage can cause this.

[RichardJ]

It sounds like you don't have any front brakes and it is probably because you never got them bled completely.
One end of your CPP combination valve has a little rubber boot. For bleeding you need to pull that boot off. Under the boot is a little pin sticking out from the metering valve portion of the internal valves.

The metering valve will not open and allow fluid to flow to the front disc brakes until the pressure reaches about 35-40 lbs. If there is air between the metering valve and the MC piston for the front side, that air will compress and allow little or no fluid through the metering valve. It may allow enough fluid to make you think you have them bled, but not enough fluid to expell the trapped air in that section of line.

The little metering valve pin must be moved to the open position during bleeding. Some pins must be pushed in on some combination valves and on others it must be pulled out and held while bleeding.
I looked all over CPP's web page and I don't know which type they sell.

Below are the tools used to aid you in holding the pins while bleeding. You just have to figure out which type you have. You don't need the tool if you have a helper.

Bench bleeding makes a lot of people feel warm and fuzzy, but it will not help you. A lot of cars have the MC mounted nose-high and will trap air if mounted dry. On a level mounted MC, the air will escape into the reservoir on its own. In your case, you've driven down hill enough times by now, any trapped air would have long since escaped from the nose of the MC, into the reservoir.

[leddzepp]

Lee: here is a step by step assembly guide I used when I redid my brakes: http://www.pbase.com/nufsed/drumbrake

[Lee H]

Brad & Richard, thanks for the suggestion on rebleeding, I will before I do anything else. With the old booster and master I did this several times, with the tool in place on the new, stock proportioning valve. When I installed the new CPP kit I followed their extensive install and bleeding procedure, twice with no change to the rear locking up. I'll rebleed, and if that doesn't help maybe I'll take it to a shop and have it power bled to rule out air in the system.

[Lee H]

Originally Posted by leddzepp

Lee: here is a step by step assembly guide I used when I redid my brakes: http://www.pbase.com/nufsed/drumbrake

Thanks Scott, that should help me confirm it's going back together correctly.

[hamjet]

I have drum/drums and mine does it if I don't drive it for a couple of weeks, then they'll lock up till the surface rust wears off the rear drums. Sure does scare ya...

[magwakeenercew2jh]

Originally Posted by Jack_71C10

I had the same problem. It turned out to be brake fluid on the shoes.

X2.

New shoes. No lock up.

[NGN]

This might not be your problem but , are the rear shoe's on right. When I got my 67 home the rear driver side started locking up. The PO had said the brakes were new all around, well as it turn out the shoe's were on wrong. At first I thought I had no brake lights and people were locking up there brakes , but then I seen the smoke from the rear tire when I hit the brakes. Scary at first , laughed when I got home. Check out all the brakes on all four and replaced the rear with new again , right this time.lol

[AirSpeed]

Had the same prob on my 72, turned out to be gear oil leaking on the shoes. Replaced wheel bearings and seals and works fine now.

[BigWheelBob]

clamp off the rear line, then check front brakes.

[RichardJ]

>>When I freshened up the truck last year I replaced "all" braking components<<

>>a recent aggressive braking action brought it to my attention. The rear shoes are nearly worn out after just 8K miles.<<

That clearly sounds like Lee H can determine the difference between new shoes, worn shoes and contaminated shoes.

It is possible the shoes were flipped rear to front and went unnoticed, but the short shoe on the rear would have all the wear and the long shoe on front, would have considerable less wear. That would have raised a flag.

I think crimping a brake line, breaks down the lining and isn't something I would consider.
Find a dirt road and see which wheels don't lock up at different brake pressures.

[Lee H]

Quick update, I removed the drums today, no sign of brake fluid or gear oil anywhere. I cleaned up everything with brakleen and verified all components were installed correctly, including primary and secondary shoes. The drums look very good, no glazing, no signs of hot spots or checking, no grooves. Shoes look OK, no glazing, about 1/4 the thickness of the new shoes, bonded not riveted, and about 1/4" narrower then the new shoes. New shoes are top of the line Wagner's, riveted, not bonded. The old shoes were Raybestos.

I'm going to have the drums checked for roundness and turned if needed and will swap out the shoes over the next few days.

Much thanks for all the advise, Lee

[Lee H]

Update: Drums check out fine, did not need turned. Installed new shoes, adjusted out to a very light drag then one click back. Test drove and braking system working normally now. Rears no longer locking up, even on hard braking. Fronts appear to be working fine, when backing up and hard brake application the fronts will lock up, did not do that before. Chalk one up to a defective set of shoes. Another case of something looking good but in reality not.

[Longhorn 70]

great news Lee. You know you're going to make me ask my wife for permission and get under the truck.