Help! C20 Disc brake conversion problems.

[Dtech]

So here is the low down, I did a front disc brake conversion on my 1969 C20 and the disc brakes are as bad as the drums were.

I found a 1977 c20 and pulled the spindles, discs, steering linkage
New calipers and brake pads
Used rotors.
Used p valve
New Master for 77
New brake lines for front brakes.
Correct hoses.
New ball joints upper and lower.
Kept original A-arms

Got the swap done. the truck stopped somewhat better, but when I put a ton of brick in it, the stopping power is horrible. When the pedal is pushed hard enough, only the front right tire locks up... barely with the truck empty.

I kept the stock 69 pedal and Brake booster and shaved down the plunger to fit the 77 master cylinder.

Bled the brakes completely.

I'm thinking the brake booster/pedal setup is to blame and not pushing far enough and therefore not building up sufficient power.

If I'm correct, what is my BEST option to handle this. Would I see better results with a 71-22 pedal booster set up. Or is there and ever better option. Ill replace the p-valve new but I don't think this is it. its just something I shoulda done in the first place

I REALLY need help because all this work is for nothing at this point. And the truck is a work truck.

Thanks

[Warrens69GMC]

I put a S10 dual diaphram booster on our 69GMC with Drum/Drum. It will throw you thru the windshield

[lolife99]

I did this same swap years ago on a 69 suburban. (1/2 ton)
My donor was a 76 1/2 ton truck.
I replaced basically all the same parts.
I had ALOT of trouble bleeding the rear lines.
It took forever to get fluid to the back.

The answer to my problem was the brake pedal push rod.
(In my case,... it wasn't quite long enough.)
I made a temporary push rod that was adjustable, so I could dial in the length.
Once I had the correct length,... I made a new push rod.

I also went from manual drum brakes,... to the '76 power brake set-up bolted directly to the firewall.

[Warrens69GMC]

Originally Posted by Warrens69GMC

I put a S10 dual diaphram booster on our 69GMC with Drum/Drum. It will throw you thru the windshield

As with the previous poster, we had to extend the brake pedal rod as well

[Dtech]

Warrens69GMC, That's what I want to hear!!! Please if you can send me pics or a write up on what you did year etc, I'm all ears... or eyes.

Lolife99, how were the brakes after that?

[Warrens69GMC]

I'll see if I can get you some pics tommorrow..

[Dtech]

Alright thanks!!

On the rod extension. When I shaved it down, cause it came out of the 69's booster, I cut it so that it would just touch the master's plunger and just touch the boosters contact point. Did you have to pre-load the rod against the booster or did you do what I did?

Thanks

[Warrens69GMC]

just did what you did. I used all thread between the brake pedal and the rod that goes to the bracket that the power booster goes to.

[Dtech]

Okay cool. If your S10 booster swap is not way too hard Ill probably go that route. But first things first, I'm going to bleed the heck out of the brakes... again.

Thanks

[Warrens69GMC]

I just used the stock PB bracket, and added the S10 booster.

[Deenice21]

what year s10? can i find at rockauto.com or just hit up a jy?

[Dtech]

Sounds real simple. I guess the dual diaphragm, as opposed to a single, really helps put pressure on the Master. I'm gonna completely bleed the brakes once again and see what I got then make sure the pedal is not bottoming out againts cab or brackets. If that doesn't do it. I'll throw that booster in. Thanks for your help.

[Dtech]

So taking Warrens advice, I bought an 85 S10 brake booster and installed it. The brakes are a lot better and more responsive. The fronts lock up but the rears do not. So I am going to tear off the rear brakes to see what I can see.

Here is what I had to do to make the Booster work.

The shaft coming out that hooks up to the brake pedal assy is too long by 2'.
So I cut mine down threaded the shaft installed a long nut and then I came across a machine eye bolt with the right diameter and thread count on it. I then installed it on the rod with the long nut in between and got the measurement I needed.

Next I had to drill out the top 2 holes on the attaching brackets because the studs did not fit. The 85 brake booster's studs are all metric 10 - 1.5 so I got some new hardware and bam!! install done



Note the

[Deenice21]

thanks for the pics.