Disc Brake Conversion Problems on my 66 C10
 

The equipment: 77 Spindles, 77 C10 Master Cylinder, 82 Corvette Booster, Captain Fab Booster bracket. Booster and MC have the correct shallow intermediate pushrod. POL Factory Proportioning valve. All new hardlines and rubber lines in front. Rear factory drum brakes rebuilt 2 years ago.

Everything went together fine. Bench bled the master, noticed the small (rear) chamber was harder to bleed than the front. Ended up tilting the MC in the vice to get the rear chamber to bleed fully. Bled the brakes the old school way, noticed several leaks, so repeated with a Motive power bleeder. Bled with the centering tool in the proportioning valve. All wheels are flowing clean fluid, no problems. No more leaks that I can see.

The problems:

1. When applying the brake pedal, it feels a bit spongy for the first third of travel then firms up. When applying the brakes, the engine feels like it idles down a very noticeable amount, its a stock 350 crate motor. 11/32" Vacuum line run to the booster from the rear brake port of the Edelbrock Carb. A booster should not affect engine idle?

2. When applying the brakes, I had the truck in the air, wheels off, and the rear brakes do not stop the rotation of the wheels, even with a FIRM push on the pedal. Rear drums adjust to have a light drag all the way around when turned by hand. Turning the front wheels and then having my fiancé apply the pedal, the fronts seem to be working fine. Rear passenger drum gets very warm.

Shouldn't the rear brakes be able to stop the rotation of the rears, even with the truck in gear? Or is the proportioning valve sending so much bias to the fronts that it wouldn't?

I'm beginning to suspect i got a bad POL proportioning valve. I bought it last year, heard they were being plagued with some bad ones?

Really stumped here, just want this project to be finished...any help greatly appreciated!

Re: Disc Brake Conversion Problems on my 66 C10
 

With the pedal applied you should not be able to turn the rear wheels at all. Do you have the rod from the pedal to the booster adjusted to the right length. I had problems with my 65 which had stock brakes and it ended up being the rod was too short. So I adjusted it and then the brakes worked correctly. If you adjust it too much the brakes will not release after a few times of applying the brakes. It was a trial and error adjustment for me

Re: Disc Brake Conversion Problems on my 66 C10
 

Master Power Brakes sells a tool for the adjustment of the pushrod you are speaking of, if I remember correctly that dimension is .015, its well worth the money, you can do the adjustment before it’s even on the vehicle…

Re: Disc Brake Conversion Problems on my 66 C10
 

If the rod is too long the brakes will not release. My friend smoked his new brakes because of this issue. So if yours is not the correct length it would be too short.
With so many different parts your issue could be some where else? Even with a rod installed that isn't the correct length the fronts are working. And the system bled. I suspect some other issue.

Every thing is new but the rear flexible line. I would confirm it's condition or just replace it before moving one.
Best of luck.

Re: Disc Brake Conversion Problems on my 66 C10
 

I have a new rear flexible line to install, but the rears were working with the 4 wheel drums, and nothing on the rear system was changed at all? Could swapping the front brakes create issues with that hose in the rear?]

Re: Disc Brake Conversion Problems on my 66 C10
 

I'd assume that if the rod were too short, the front brakes also wouldn't work? The rod is definitely not too long, as I can turn the front rotor by hand, but can't after the brakes are applied.

The pushrod between the booster and the master is correct. I confirmed with CaptainFab, they are both shallow configuration, so are a match.

Re: Disc Brake Conversion Problems on my 66 C10
 

Engine idles down when brakes applied? Is the new booster bad? Pump the brakes till hard and keep pressure on the pedal as you start the engine. The pedal should give a little bit.

If the booster is okay do you need to road test it to see how the front brakes are working?

Re: Disc Brake Conversion Problems on my 66 C10
 

OK, one problem (not really a problem after all) solved. The engine "idling down" was just the engine, in gear, fighting the rear brakes. The pedal is hard when off, started it up and the pedal gave a bit, so the new booster is working. When the engine is idling, not in gear, applying the brakes has no effect on the engine.

So all that is left is to figure out why the rear brakes are not locking when brake is applied. I'll rebleed, swap that rear line out/ rebleed the rears, re-Bench Bleed, then change out the Proportioning Valve, in that order and report back...

Re: Disc Brake Conversion Problems on my 66 C10
 

I've not done it but the newest method for bench bleeding is by using a syringe to push fluid through the master cylinder port backward into the master to purge the air.

Re: Disc Brake Conversion Problems on my 66 C10
 

Is there any need to use the special tool that keeps the plunger at the end of the Proportioning valve inserted, while bleeding? I ran across that last night while searching for an answer to my issues.

Re: Disc Brake Conversion Problems on my 66 C10
 

There is a small tool available at many retailers that screws into the center port on the distribution block that keeps it balanced so the brakes can be bled correctly….

Re: Disc Brake Conversion Problems on my 66 C10
 

Yessir, I used that tool. Does this tool do basically the same thing?

Re: Disc Brake Conversion Problems on my 66 C10
 

UPDATE: So, thinking more about it and discussing with my friend, he said he didnt think the rear brakes would hold enough to stall the engine in Granny 1st or 2nd gear. So I ran the truck up to 60mph in 4th gear, confirmed the wheel was spinning, and hit the brakes. Wheel stopped and engine stalled (wasn't holding in the clutch). Basically a classic case of me overthinking something and seeing a problem where none exists. (Yes, I'm a moron, lol).

So, bolted on the wheels and took it for a test spin, the brakes work PERFECTLY. Like pitch you through a windshield perfectly. The track straight and true, and RIGHT NOW. All I need now is a front end alignment and I'm good to go. Thanks to all that offered help here and Captain Fab for a great bracket and follow on support!