Rear disc conversion issue

[84 400]

The conflicting info out there on this sucks!

Info on the setup:
77 k10 with axles out of a 85 K20 HD.
Stock 77 master and vac brake booster.
Stock 77 proportioning valve
Stock 77 front calipers
1976 caddy elderado calipers on the rear.

The problem:
After multiple bleedings the trucks pedal goes to the floor. Fronts are still working but the rears are not. So what could be causing this? I have read online about disc proportioning valves, residual valves, a adjustabe proportioning valves. I can't afford to just throw parts at this. Is it possible to gut the proportioning valve?

Any info would be great as I need my truck!

[mgchevyparts]

Sounds like a bad master. Did you ratchett the calipers so the pads are nearly touching the rotors BEFORE you bled them? If you have too much space between the rotors and the rear caliper pads you can't bleed them. Also your bleeder screws are in the up position, right?

[GMC Guy]

good luck man... i can tell you that a corvette style disc/disc master cylinder + brand new booster + a disc/disc proportioning valve + all new stainless braided lines + blackbirds rear disc kit = super soft pedal with truck running. That's right, i literally replaced every component and it still doesn't function right. So far, rear disc conversion has not been worth the money or headache. I'm sure it'll get figured out somehow, and i'll chalk it up to "learned the hard way"

[BLE 'BURBAN]

Here's something I found last thursday (April 11th, 2013) on classic performance parts website.

[84 400]

Yeah bleeders up. i pulled on the ebrake and it has the pads almost touching then i bled them.

The master did run dry while bleeding. Could that be the issue?

[Keith Seymore]

Originally Posted by 84 400

The master did run dry while bleeding. Could that be the issue?

Yah.

K

[N2TRUX]

Originally Posted by 84 400

Yeah bleeders up. i pulled on the ebrake and it has the pads almost touching then i bled them.

The master did run dry while bleeding. Could that be the issue?

Very much so. I would pressure bleed, or gravity bleed and see if that helps

[Jonboy]

Do you have to reset the prop. valve if the master ran dry? I am having a similar problem on my '74. There is amonst no pedal at all, and the front was dry. I don't see any leaks at the wheels (or anywhere else).

[silvereagle07]

Remove your master cylinder and bench bleed it. Install it and refill with fluid. If the metering valve is bound the light in the cluster is probrably on. Have an assistant apply pressure to the pedal like your bleedig the system and crack open the line on the metering valve for the front or rear. Open the line to whichever is not getting fluid. It will reset the valve. Close the line when it resets. Then gravity bleed brakes and pressure bleed brakes. Keep the master cylinder filled during the process. There is a tool available to keep it in positon when bleeding the system where you remove the switch and insert a valve locking tool. Then install switch when done bleeding the system.

[84 400]

Okay so the rear running dry would cause it to not build pressure? I mean fluid is still being pushed out as was air. now it is just straight fluid but the pedal is going to the floor. If i have to bench bleed should i swap in a vette master to have equal resi fluid capacity or should the factory master be good enough?

Thanks for the help!

[silvereagle07]

A bigger reservoir and master cylinder would be ideal. The calipers should require more fluid than the wheel cylinders.

[BLE 'BURBAN]

Just today I ordered a disc-disc gm master cylinder for a 1997 GMC JIMMY "TYPHOON" RPO CODE JC1 thru work for the disc brake conversion I got from Blackbirds custom trucks.

I also found this from Classic performance products......(post #4 above)



You'll need to swap out both the master cylinder and the prop valve in order for your brakes to operate properly.

Currently all I've done is swapped out the rear brakes to discs with the kit I bought from blackbirds (No E-Brake style) The pedal is a bit spongy but all I have to do right now is a quick 1-2 pump on the pedal and I can stop a-helluva-lot shorter than ever before. But once I get the M/C & prop valve replaced/modded,THEORETICLY my brakes should function properly....... Time will tell.

[INSIDIOUS '86]

From what I've read the brake line size between the front and rear are different with the front being smaller diameter lines to move more fluid that and I don't belive that when you push the pedal down that the rear gets pressure untill you reach a certain point in travel which is why disk disk master cylinders are necessary.
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[UATahoe]

i am confused here too. Getting ready to swap my 87 rear drum brakes out and use discs but cant figure out if i need just an adjustable prop valve or if i need it and a new disc/disc master cylinder? And it seems like finding a disc/disc cylinder that works is a pain as well. I just want to make sure i do it right if there is such a thing. LOL

[83GMCK2500]

To the OP, have you tried THIS PROCEDURE?

[83GMCK2500]

I am going to convert my truck to 4Whl Disc when I swap rear axles. I plan to use a GMT-400 master cylinder of the correct size per GVWR (example: Typhoon MC listed above is only for 5,500 lbs) and if I need I will use an adjustable proportioning valve at the MC for the rear brakes. I will keep the stock proportioning valve in place. For example a MC from a '93 K3500 has a 1-1/4" bore F/R and is rated for somewhere in the neighborhood of 10,000 GVWR.