Rear discs won’t lock up after frame off build

[z71se]

Hey guys I have an issue that is just really stumping me. I’m using a 4-wheel Wilwood kit completely spec’d by them for 15” wheels. I plumbed the whole truck with 3/16” line and there are no leaks or air bubbles coming from any of the fittings or bleeder screws.l anywhere in the system. The master(s) I’ve tried have been bench bled correctly and fluid volume is clear and without bubbles. I’m running a Wilwood “combo” splitter/ adjustable proportioning valve and I’ve tried adjusting the bias every which way... makes no difference to the rear lockup problem. With the rear axle lifted, and the truck in drive or reverse, I can slow the rear wheels a little but I can’t bring them to a complete stop. I haven’t driven it yet because I’m sure the truck would lurch forward and might be unstoppable given the inability of the rear brakes to stop the truck.

I’ve tried both a stock replacement 1.125” master as well as a C3 Corvette disc/disc 1.125” master (the bore size that Wilwood recommends for my calipers). Both had identical results. I don’t think the master nor bleeding is the issue because two OEM masters had identical results. Could the problem be that I’m using 3/16” lines with these two OEM masters? Obviously the C10s had 1/4” rear lines but I’m not sure what C3 Corvettes had. I’d like to avoid spending $300 on a Wilwood master if it’s unnecessary. They’re gawdy and stick out in the engine bay.

[Dead Parrot]

Not an expert

Most of a PU's braking comes from the front. The systems are designed to avoid locking the rears up. The early ABS on PUs were usually only on the rears for that reason.

I would think the only difference in 3/16 vs 1/4 would be a minor lag in the brakes applying. Probably not noticeable without special instruments. The brakes don't move a lot of fluid when applied.

Are the fronts clamping down tight?

Does that prop valve have any kind of reset button? Some prop valves have an internal valve that will lockout part of the system if there is a leak and there is a magic process to reset it. Possible that got set during the bleeding process.

[z71se]

Originally Posted by Dead Parrot

Not an expert

Most of a PU's braking comes from the front. The systems are designed to avoid locking the rears up. The early ABS on PUs were usually only on the rears for that reason.

I would think the only difference in 3/16 vs 1/4 would be a minor lag in the brakes applying. Probably not noticeable without special instruments. The brakes don't move a lot of fluid when applied.

Are the fronts clamping down tight?

Does that prop valve have any kind of reset button? Some prop valves have an internal valve that will lockout part of the system if there is a leak and there is a magic process to reset it. Possible that got set during the bleeding process.

Yes, fronts do clamp down tight. Prop valve doesn't have a reset button. It's just the splitter/ valve combo Wilwood offers:

[PGSigns]

Sounds like air in the system. I would pressure bleed it with the adjuster all the way open. I know this has come up here a couple of times but are the rear calipers on the correct side with the bleeders up?
Jimmy

[z71se]

Originally Posted by PGSigns

Sounds like air in the system. I would pressure bleed it with the adjuster all the way open. I know this has come up here a couple of times but are the rear calipers on the correct side with the bleeders up?
Jimmy

Thanks for your post. Yes I have bled with the valve all the way open and all the way closed. The Wilwoods are reversible. There are bleeders on top and bottom.

[PGSigns]

I think you have some air trapped.
Jimmy

[Dead Parrot]

I know from experience that my CUCV will stop fine on just the front brakes. One of the push rods escaped from a rear wheel cylinder and let all the fluid escape from the half of the MC for the rears. Truck still stopped fine while driving on the back 40.

Possible the system is working as intended. What does Wilwood have to say about it?

Otherwise, I have to go with the folks saying air in the rear system. Once the fronts clamp tight, the MC can't move anymore and the air bubble keeps the rears from fully pressurizing.

[PGSigns]

One more thought on this. Did you move the pushrod down on the pedal an inch and is the pushrod long enough? You may not have enough travel to get it bled correctly.
Jimmy

[Kudzupatch]

Originally Posted by z71se

I’ve tried both a stock replacement 1.125” master as well as a C3 Corvette disc/disc 1.125” master (the bore size that Wilwood recommends for my calipers). Both had identical results.

Basic trouble shooting says that it is not the MC unless they have identical issues. That is highly unlikely.

I would start working my way backward. It might be a defective valve. Blockage in the line or air. Me, would lean toward the valve since the adjustment made no difference if you are sure you did a good job bleeding the lines. You should see some difference there I would think.

And with the car jacked up if you can't stop the rear wheels do have a problem!

[Koz1]

May need to lengthen pushrod?