Master Cylinder Help

[BanjoDude]

Hi guys,

I have my grandpa's old '69 C-20. The guy I bought it from had done a nice restoration, but apparently didn't touch the engine bay. I've been cleaning up the wiring (from an old security system he had installed at one point, etc.) and am going to slowly clean up the engine bay. The master cylinder and proportioning valve are in pretty rough shape. I'd love some advice.

The master is marked with 29910 (if I'm reading correctly).

The proportioning valve is marked with "M-R" on the top and "9-6" on the side.

The truck is still the original drum/drum braking configuration.

Should I replace the master cylinder and valve? If so, what would you recommend? I'd rather cry once / buy once and get good stuff.

Am I better to send them off for a rebuild? If so, where?

Anything else I should do while I'm at it? The booster is in good cosmetic shape, and brake power is very good, though the rears lock too easily in my opinion.

Thanks in advance for your help!

Wes

[LockDoc]

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If everything is working and the master cylinder is clean on the inside I would just leave it be. You can clean the master cylinder and valve up with a small wire brush and put a couple of coats of paint on them with a small paint brush.
.

[BanjoDude]

Originally Posted by LockDoc

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If everything is working and the master cylinder is clean on the inside I would just leave it be. You can clean the master cylinder and valve up with a small wire brush and put a couple of coats of paint on them with a small paint brush.
.

I decided to take it apart to inspect it before painting it, and I'm sure glad I did. It was completely full of gunk and the bore is badly pitted, so rebuilding is not an option unless I send it out to be sleeved. I can't believe the brakes even worked with things in this condition. While I was at it, I also tested the proportioning block, and it's seized up also.

Any recommendations on the best master to buy? I've dealt mostly with Classic Parts up until now, but would like to buy the best thing I can so I don't have to do it again.

Thanks!

Wes

[Rust_never_sleeps]

That is a "distribution block", not a combination valve. The only valving in there is the differential pressure(aka shuttle) valve that closes a leaky front or back circuit.

You could add an inline adjustable proportioning valve(APV) to tame the rear brake grab, and allow you to adjust for different loads

[Steeveedee]

I bought my master cylinder from my local NAPA, about a mile from me. I also use an inline adjustable proportioning valve. Turns out mine's wide open, as there is plenty of weight on the back tires.

[vics stuff]

I had found that Summit Racing had in stock a NOS Delco master cylinder , which I bought. My original one was in the same condition as yours. I even found a NOS rebuild kit , which I was going to use and found my master cylinder to be beyond repair
Vic

[mr48chev]

I'll just say this, I did brakes for a living for a number of years and rebuilt untold single piston master cylinders and wheel cylinders but have NEVER HAD any luck rebuilding dual piston master cylinders. I tried it a couple of times when I thought I was going to save a few bucks and ended up spending more when I bought a new master cylinder after the one I put a kit in didn't work out.

[BanjoDude]

Originally Posted by vics stuff

I had found that Summit Racing had in stock a NOS Delco master cylinder , which I bought. My original one was in the same condition as yours. I even found a NOS rebuild kit , which I was going to use and found my master cylinder to be beyond repair
Vic

Would you happen to have the proper Delco part number? I'd love to find a NOS original.

[Rich69shortfleet]

Honestly, if you are tearing into it that far, now would be the time to rebuild the whole system with the 71/72 style disc/drum setup. The conversion is pretty simple and the truck will stop MUCH better. The braking improvement is hugely helpful too if you ever tow or haul with the truck.

[BanjoDude]

Originally Posted by Rich69shortfleet

Honestly, if you are tearing into it that far, now would be the time to rebuild the whole system with the 71/72 style disc/drum setup. The conversion is pretty simple and the truck will stop MUCH better. The braking improvement is hugely helpful too if you ever tow or haul with the truck.

That thought did cross my mind. It was my Grandpa's truck, and I'm kind of a sap for originality, so I decided against it. I'll never tow with it, so that's not a factor - it's just a fair weather ice cream getter.

[HO455]

I would replace the wheel cylinders too. Highly likely they are in the same condition inside as your master cylinder is. Then you will only have to bleed the brakes twice.

Twice??? Here's what I mean.
Bleed them once with the new master cylinder and the old wheel cylinders to flush all the gunk and bad fluid out without any chance of getting gunk in the new wheel cylinders. Then replace the wheel cylinders and bleed them with fresh fluid in the lines.

P.S. replace all the hoses when you replace the wheel cylinders.