Rear Disks Staying Tight?

[72longbed]

I have converted my 72 K20 to rear disk brakes and still use the factory 3/4 ton master cylinder and prop. valve. I've been noticing that the rear rotors have been extreemly hot after even the shortest ride to the store or 1 or 2 miles anywhere. The front rotors are not hot at all when I check all four rotors. I have put the truck up on the lift and don't really notice a lot of 'drag' when rotating the rear axle without the truck running. (The 90WT gear lube that leaks from a bad axle seal on one side is actually smoking where it contacts the rotor!)

What I'm wondering is could the 2WD, drum/disk setup be putting undue pressure on the rear circuit of the brake system? If so, is there an easy way to fix this without replacing the master cylinder? I have heard of a disk/disk proportioning valve, but my understanding of prop. valves is that they do not hold line pressure on either circuit, but only are used to make sure that you still have front or rear brakes in the event of a failure in one circuit or the other?? Is this wrong?

Does anybody know what I can do here to fix this problem without replacing the M/C??

Thanks for your help,
72longbed

[72longbed]

Any ideas out there?
ttt

[VetteVet]

This text is copied from the following article.

http://www.hotrodheaven.com/tech/bra...kes1_index.htm





If you experience a brake lock up after a few applications of the brake pedal, it is directly related to a residual valve retaining the brake fluid within the lines and not allowing the fluid to flow back to the master cylinder. The problem is either the wrong residual valve being used, a drum brake master cylinder being used on disc brake calipers, a inline residual valve plumbed in to the brake system with a built in residual valve in the master cylinder or a defective residual valve.

OEM tandem master cylinders will have a residual valve built in when there is a drum brake application. That is why it is important to buy the correct master cylinder based to application. Yes, you can remove the residual valve from the master cylinder, but often the reservoir is to small and it does not hold enough brake fluid for the disc brake application. So great care must be taking when using a modified master cylinder. OEM tandem master cylinders were designed to be cheap. Careful consideration should be made when selecting the master cylinder, because of the high volume of brake fluid required and pressure for the disc brake application. OEM tandem master cylinders do not produce the same volume as two side by side master cylinders. Remember the application is stacked one in front of each other so you have a limited travel and volume to work with.

[71K-20]

Did you buy a kit of build this setup yourself? I am doing a brake job on my 71K20 with Eaton rearend and need drums that I cannot find so I am investigating the rear disc option. The issues that Velvet Vet brought up came to my mind, but when I called Blackbirds Custom Trucks in Spokane they told me to just bolt on the calipers hook it up and in two hours your done. Sounds to good to be true.

[cdowns]

actuallyon these trucks i believe the residual valves are built in to the combination valve that most people with these trucks mistakenly call a prportioning valve
go to speedway motors catalogue they have a great diagram of what is required for a brake system to function properly

[72longbed]

Went to Speedway Motors website, nice website but looking at new parts is not going to help me figure out whats happening with my system right now. That other article does mention something about an 'internal' residual valve in the factory MC? Does the MC in my truck have one of these for the rear drum brakes? Can it be disabled or otherwise removed?

Thanks for the help everybody.
72longbed

[qksilver]

yep

[wibilly]

i've had rubber hoses on front discs go bad and cause calipers to not release. i'm not saying this is your problem just a thought. keep us posted when you solve the problem. i'm adding discs to my eaton rear as soon as my brackets get here, should be this week but truck is going to be disabled for a few months yet

[woodridge]

To verify the problem is in the master cylinder, apply the brakes, release the pedal,see if the rears are locked. Then loosen the line at the mc for the rear brakes and if the brakes release it is most likely the check valve in the mc. You can remove the residual check valve in the master cyliner by carefully screwing a self threading screw into the invervted flare seat then with two flat screwdrivers prying the seat out, the check valve is located underneath, it's rubber and has what looks like a duck bill on the end of it. Once removed, reinstall the flare seat, bleed and theoretically, Wa-La it's fixed.

[Mudder]

I have used a drum/drum master from a 67 for about 8 years and have never had any problems like you mention. I have checked from time to time the heat and have never felt anything more than the normal warm temp of a hub/wheel. I do change the back calipers regularly so that they don't wear down till there isn't enough fluid to push them. At some point I would like to upgrade but for now they are fine.

[cdowns]

speedway motors I TOLD YOU TO LOOK AT THIER CATALOGUE NOT THIER WEBSITE all the info is there in hard copy

[Blazer1970]

I believe that this can also be caused by having "too good" rear brakes. Did you rebuild your front brakes at the same time? What will happen is too much of the braking energy is transferred to the rear axle, and since the heat that is generated is directly proportional to the braking energy, they will get hot. Disc brakes are designed to operate at very high temperatures, and this is usually not a problem. New brake pads also seem to generate a lot of heat during the break-in process. Any effect like this will also be more pronounced on a short trip where the brakes are used frequently over a short distance. I have seen the same thing when I did rear brakes on my 2001 2500HD (factory rear discs) without doing the fronts at the same time. There is a lot of pad left up there, but I am sure they are not working up to their potential due to some glazing over time.

[svgould]

What kind of rear calipers are you using?? If using the 78-85 Cadillac rear calipers with the built in parking brake (like many kits use) then you can use the stock MC and prop valve. If using a caliper that was originally used for the front of some vehicle then you will have to swap the MC and prop valve. This is because the rear cadillac calipers have a spring that pull the pads back off the rotor when you let off the brake, whereas the front calipers don't. There is a website out there that explains it all, but I don't remember right now.....it called steel tech solutions or something like that.

[svgould]

I found the website that explains it.........http://steeltechsolutions.com/RDBINSTRWORKPAGE.html
it is down torwards the bottom!!

[kwmech]

You can also install a mid seventies 4 wheel disc corvette master cylinder and remove the prop valve all together

[72longbed]

KWmech,
thats what I want to do! Please provide any and all info on which one would work with the 3/4 ton vac. booster that is behind the stock MC. I like the idea of eliminating the prop. valve and getting a disk/disk MC at the same time. How does it function without a prop. valve? Will I be giving up any safety?

Thanks,
72longbed

BTW -- I'm using 1976 K20 front rotors and calipers/pads on the back now.

[72longbed]

cdowns,
I don't have their catalog, so I can't look at it. But thanks anyway.

kwmech,
anymore info on the Corvette MC??

Thanks to all,
72longbed

[RainFade]

Get a disc brake combo valve.
The disc/drum one holds pressure to the rear drums so they energize quicker and now that you have disc's back there they are dragging.
They'll get thermonuclear hot on just a short drive !!

[72longbed]

RainFade,
I hear what you are saying, and Yes, the rotors are getting hot enough to make 90 wt gear oil smoke back there, but it seems that everything is pointing to needing more than just the disk/disk prop. valve? Do you agree that my factory 1972 MC has a built in residual valve that is holding 10-12 lbs of pressure on the rear calipers? I'm waiting for more info before I decide which way to go on this fix. Whether to just replace the MC with a 1975Corvette disk/disk MC and eliminate the prop. valve or go into the factory truck disk/drum MC and start playing around inside of it?

It would be good to hear from somebody who has done this fix and knows the pros and cons of each way.

This is good info from everybody and I appreciate it!! Keep it coming!
Thanks,
72longbed

[RainFade]

Well lemme put it this way I re used my 71 MC on my conversion after reading the residual was in the combo valve.
I used a combo from an eldorado with 4 wheel disc.
Had to dial the pressure down on the rears with an adj prop valve as they tended to lock too soon.

Good luck and stay at it as it IS worth it

Brian