Front axle swap master cylinder/prop valve issue

[ReeseBobby]

I'm in the process of swapping out my front axle on my 1970 K20 to a disc brake axle. I purchased a master cylinder / proportioning valve from Summit and attempted to install them on my black brake booster. However, the booster does not correctly mate to the MC. The booster has a long rod in the center that does not attach to the new MC. The hole in the MC is too small.

Additionally, the prop valve fittings do not mate correctly with the original brake lines that fit the old prop valve. I was able to get the truck operational by leaving the old MC and Prop valve on the truck, but I know this is not optimal. Under hard braking, I can lock the rear drums up, and while the truck stops better in general, I don't feel the disc brakes much more than I felt the drums.

Will the drum/drum booster not work with disc/drum, or am I just using the wrong MC / Prop valve? I purchased the disc / drum setup from Summit and thought it would just be a slap on and screw in. Any help would be appreciated. Thanks!

[ReeseBobby]

Ttt. Anyone?
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[airdale94]

You can get a reman Cardone brake booster and master cyl. from O'Reilly's for a 72. You can try to souce a stock prop. valve or just make your own lines. There are also aftermarket booster, mc, prop. valve (72 style) available on E-Bay and other vendors, you get everything. I think a 73-87 prop valve is the same as 71-72.

[Erl]

I am having some of the same issues. On the 3/4 ton the front MC port goes to the rear proportion valve port. The rear MC port to front port on pro valve. These lines are available from In-Line Tube. I believe the GM 71-72 proportion valve is the same for 1/2 and 3/4 ton, but not 100% sure. I used a rebuilt 72 booster and a new 72 master cylinder.

Erl

[ReeseBobby]

I guess I'm confused on the proportioning valve ports for the brake lines. I only have a single rear and a single front brake line, but the prop valve has two ports for front brakes and one for rear. Can I operate the front discs with just the one port? Thanks.

[airdale94]

I have seen posts like this before and they say to just plug the front one. I think the 73-87 style has 2 front ports.

[RichardJ]

>> fit the old prop valve. I was able to get the truck operational by leaving the old MC and Prop valve<<

Your drum/drum brakes DID NOT have a proportion valve. That old piece is nothing more than a distribution block with a switch to turn on the dash "Brake light" to indicate a brake failure.

If you have a new valve with five (5) ports, you still don't have a proportion valve. You have a "Combination Valve" that includes a proportion valve.

There will be one Inlet port for the rear brakes and a corresponding Outlet port for the rear brakes. Those two ports comprise the Proportion Valve portion of the Combination Valve.
The other three ports ( one Inlet and two Outlets ) are the Metering Valve portion of the Combination Valve, That is for the front disc brakes.

[ReeseBobby]

Thanks so much. You guys are great.
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