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.