Residual pressure valve for drums

[Skip Fix]

Not a total disc question but front disc conversion rear drum question for the many brake experts here.

So having a debate on my Pontiac board about a front disc conversion with keeping the rear drums and if a RPV is needed or is used for drums still. One guy said GM stopped using RPV in 72 but I could swear the difference in the 1 1/8 Disc/drum vs 4WD 79-80 MC was just the RPV for the drums and a different prop valve. I know early GM products with discs like 67-70 ish used a special valve to delay the discs.
http://www.classicindustries.com/product/1969/camaro/parts/f68691.html

Trying to find new replacement MC for rear drums with a RPV is getting hard as they all seem to be front and rear no RPV.
Everything I've read is drums even when the MC is not below brakes like street rods needs a RPV.

[dontlifttoshift]

All drum brakes require a RPV to keep the cup seals in the wheel cylinders tight against the bore. There are no exceptions to that rule. Sometimes the RPV is in the combination valve.

If you can not find a master with one, plumb one in.

[ccmc]

The residual pressure valve on a 1969 Camaro is inline under the drivers seat area. They are also available that connect near the booster, like an add on proportioning valve

[CliffsBlueCamaro]

For whatever reason, my car was missing the residual pressure valve. When I plumbed one into my disc/drum combo the pedal feel got exponentially better.

No exaggeration - it was a night and day difference. This is the one I used. (https://www.summitracing.com/parts/wil-260-13784/overview/)

[ctcz28]

The residual pressure valve on a 1969 Camaro is inline under the drivers seat area. They are also available that connect near the booster, like an add on proportioning valve

The valve under the drivers seat on the subframe on a '69 Camaro is a proportioning valve. The residual pressure valve for the disc/drum combo is located in the rear brake port in the MC for the power brakes with the 1 1/8" bore. The barrel shaped valve located just under the MC is a metering valve, or hold off valve, for the front discs. For a more complete description check out the Camaro Research Group website at this link:
http://camaros.org/brakevalve.shtml
You should still be able to get a MC with the RPV, or add one as suggested above.

[ccmc]

Thanks for the clarification!

[Skip Fix]

Problem seems to be finding a 1" manual GM with rear RPV and a decent recess for the pushrod. It is not hard to pull a RPV out of a manual 4W drum MC.

[JEFFTATE]

Here's a little more information about the RPV.
I took mine out years ago and have had no problems with my brakes ,
https://www.pro-touring.com/threads/56851-Brake-Master-Cylinder-Bleeding