I've got a 68 junker that had AC and auto trans, I don't know the engine, but the chassis number is 12437 which indicates a V8. It has drum brakes, and no valve on the frame.
My Helm factory manual for 67, in the drum brake section, calls the valve on the frame a "Pressure Regulator Valve.
Says it was for use in AC cars - "controls the hydraulic pressure to the rear brakes resulting in the correct pressure balance betweeen the front and rear hydraulic systems".
The Disc Brake Section has different text and says the valve is located just under the master cyl, and "in the FRONT brake line".
They even show an illustration of the master cyl and valve, but it is what we call the metering valve.
-"models with disc brakes have a pressure regulator valve in the FRONT brake line just below the master cylinder. The valve controls the hydraulic pressure to the front or (rear) brakes as applicable" (they mentioned the corvette having a valve in the rear line above this sentence) "resulting in the correct pressure balance between the front and rear hydraulic systems. This valve guards against premature lock up of front or rear wheels when brakes are applied."
You can read a clip of the drum brake section of the manual manual here: http://ourworld.compuserve.com/homep...rop_manual.jpg
I've got a friend with a 69 Z/28 that had the valve and no AC of course.
I'm not disputing anything said about this valve at this point, just trying to figue out what is going on, what it does, so I can put accurate info on my brake page.
Front discs require much more line pressure to operate because they are not self engergizing. Something has to reduce the pressure to the rear drum brakes to keep them from locking first. some proportioning is done by using a smaller wheel cyl in the rear but disc line pressures are almost twice what drum pressures are, so if disc/drums work without a prop valve, the rear drums would be way too low for a drum/drum system.
It obviously isn't the metering valve attached to the master cyl, because it is in the front brake line, not the rear line.
If someone who has this valve on their car would unscrew the plug and remove the valve inside, replace the plug, bleed and test drive the car and tell us what difference it made we might learn more about it's function.
Check my web page for First Gen Camaro suspension info:
David's Motorsports page
First Gen Suspension Page
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Chev 350 Can-Am Vintage Racer
[This message has been edited by davidpozzi (edited 07-30-2001).]