69 C10 disc brake lockup

[ron lechman]

Hi.

Problem:

The front disc brakes are locking up on my 69 C10.
It was an original drum/drum system converted to disc/drum system using GM donor parts. Don't know what year the parts came from.

Situation:

After several years of owning the truck I finally spent the time to get the truck running by jumping the run wire and installing a new starter, battery and cables.
The master cylinder was empty. I filled it and proceeded to flush the system thru the bleeder fitting at all four wheels. The brakes were locked up at all four corners. I soaked the front rotors with penetrating fluid and sanded the rust off of them. Eventually I got the rotors to turn. I also removed the rear drums and cleaned up the drums and shoes. They turn freely.

I removed the bleeders and "gravity bleed" them as I had no assistance at the time. The fluid eventually flowed out clear and after closing all the bleeders I had a good firm pedal.

Yesterday I was finally going to drive my 69!

Much to my disappointment the truck barely moved.
I found that the front discs were locked up.
I removed the wheels and after some hammering and prying I got the discs freed up.
I drove the truck a few hundred yards and after applying the brake a couple of time I found the front discs locked up again.
I opened both bleeders but the front discs remained locked up.
I removed the wheels and hammered and pried the calipers until the rotors turned freely.
I left the bleeders open and drove the truck back with little braking but no more lockup.

Objective:
I still would like to use the 69 as a driver.
I don't want to throw a pile of money at it hoping to rectify the problem.
Any suggestions to rectify my problem?

My plan:
Tomorrow I am going to remove the calipers and the rotors.
I will clean and lube the caliper mounting bolts.
I'll sand and clean both sides of the rotors.
I'll clean, check and grease the wheel bearings.
I'll check the movement of the calipers.
Anything else I should do when the calipers and discs are removed?

On the forum I read something about resetting the proportioning valve?
How do I do that?


Old history:

I bought a complete 69 C10 for a parts truck but it's too nice to strip so I want to save it. I decided to use it as a driver while I restore my 68 C10. It sat for a few years. There was a "no start" and electrical issues. I did not know the engine, trans or brake situations. I only had the truck towed and never drove it.
I bought it from someone that bought it from someone etc. so a proper history of the brake conversion is unavailable. It looks like someone had installed "junk yard" used Gm parts to convert to front discs. I don't know if it had brake issues previously.

[Bigdav160]

I think the calipers are full of rust. I would rebuild or replace them. If there was a restriction in the hose; opening the bleeder screw usually eliminates the pressure holding the rotor.

[ron lechman]

Thanks for a quick response Bigdav160.
When I have the calipers off tomorrow I'll inspect the outside of the piston for rust and remove any that is present Then I'll bench bleed them checking for rust or debris coming out and how smooth the movement is.
If they don't seem right I'll go ahead and purchase new calipers.
I have a budget but NOT when it comes to safety.
Cheers!!

[geezer#99]

You got a booster too, or just manual discs.
Could be simply that the pushrod for the m/c is not adjusted right. Too tight and the m/c piston could be covering the compensating port.
Is it actually a prop valve for discs or a distribution valve for drums. A pic will help.
BTW, welcome to the forum from the west coast.

[ron lechman]

Hey geezer#99. Thanks for your quick response and welcome.
My 69 C10 has a power booster. There is a valve mounted to the MC / booster assembly that looks like the ones on 74 and up GM trucks that I've seem in the wrecking yards.
I'll take some picture and try to post them. I'm not up to speed on the computer so I'm unsure if I can. I'm still living in the time our trucks were built.
I guess I can loosen the MC to booster nuts a few turns to see if that relieves the pressure to the front disc brakes.
Cheers!! Ron

[volksworld]

rebuilt calipers are a must and are like 20 bucks...rubber hoses that are internally swollen can act like a one way valve...you can force the fluid through them with pedal pressure but they wont return on their own...about another 20 each....and if the calipers are bad theres a good chance at least one piston on the rear wheel cylinders could be frozen too...if the pedals firm and not sinking to the floor you might be able to get away without replacing the master for now if money is an issue

[lowbucktrucks]

next time it does it crack the brake line at the master cyl, if pressure on the disc goes away its the master cyl.
usually if its rust in the calipers only one will lock up, not both at the same time ( but it can happen)

[ron lechman]

Thank you for your response and suggestions.
Disc brakes on an older Chevy truck is something new to me.
I've played with drum brakes on older vehicles and replaced the brake hoses as soon as cracks were showing on the exterior rubber. I've never experienced a collapsed hose.
The 69 has been off the road for at least ten years in total so I guess investing a hundred bucks or so in new front brake hoses and rebuilt calipers would be a good investment and a good place to start my disc brake lockup solution.
The rotors look real good with little wear and the pads have lots of life left.
Cheers!! Ron

[lowbucktrucks]

Best to get brakes right, if there is any doubt R&R. Check all hoses, they can collapse and not allow presssure to release also.( They will expand when you apply brakes and collapse when you release and not allow pressure to release.
Cruising and going fast are fun, but stopping is cool!!

[ron lechman]

I appreciate your input volksworld.
It will cost a few dollars but it makes good sense.
Cheers!! Ron

[ron lechman]

Good advice lowbucktrucks. Thanks

After receiving the responses and good advice on the forum from you and others I plan to replace both front brake hoses and the brake hose at the rear diff and purchase rebuilt calipers.
I'll bite the bullet and replace the master cylinder as well.
I'll repost the outcome once I've changed those components.

Good brakes make good sense.

Cheers!! Ron

[ron lechman]

I checked the front brake hoses today and found some small cracking so I will replace both. The metal lines to them look original and weak so I'm going to replace them as well.
The calipers are removed and will turn them in as cores on rebuilt units.
The rotors look real good but I'm going to take them in to be "cleaned up" and checked for warping.

There was some debris in the bottom of the master cylinder. I'm going to remove it and disassemble to see what's going on inside. I might turn it in for a core after "playing around" with it.

I took pictures of the prop valve but don't know how to post them.