O.K. I am wanting to get a few things clarified before I get started on this conversion, Ive done this before on a 55 chevy but I used a G.M. master cylinder that someone gave me and I cant remember what it was off of??? I am wanting to use all new stuff this time and I read somewhere a 68 chevelle (drum/drum) master was the one to use is this correct? Im sure there are others if you want to share? I used a perportioning block off of a 68 truck the last time but was this necessary or can I just run my lines straight from the master cylinder to the brakes since its drum/drum? Front resevoir to back brakes? Rear resevoir to front? Does it matter? Hopefully this will also help someone else.
On my 55 Lincoln I used a generic GM m/c with a 7/8" bore for my drum/drum POWER brakes, along with a 8" booster. I used no proportioning valve, but I did use two 10psi external RPVs (residual pressure valves). Some m/c's have the RPVs built in to the ports, but not the one I used. Since I was converting from TreadleVac power brakes to a more generic and modern dual m/c, I had to INCREASE the pedal ratio in order to soften the pedal some. (TreadleVac uses a 3/4" bore with a long stroke) OH,,, the absence of a prop valve has not resulted in uneven braking... the car stops great.
What does a residual pressure valve do? What master cylinder would have that built in if it is necessary?(anyone?) Sorry for 20 questions but I want to make sure me and everyone else is on the same page.
The ten psi RPV is used in a drum brake system to prevent air ingestion past the wheel cylinder seals when you release the brakes. As far as a master cylinder late 60's GM cars had 4 wheel drum brake dual m/cyls. On some of the cylinders the residuals were installed behind the seat where the line screws in, some applications used cup expanders in the wheel cylinders instead of rpv's.You can test for the residuals by GENTLY inserting a straightened paper clip, or a small diameter piece of welding rod into the hole in the seat. If you feel something in the first 1/2 inch or so that's the RPV. If there is none in the cylinder you can add external valves in the lines.We have use Corvette manual masters and external residuals on the chevy's very succesfully. They have huge reservoirs and work with the stock pushrod and rubber boot. Mounted on the firewall, front fitting feeds the front brakes. Good luck.
My '57 has a '68 Chevelle non-power drum/drum dual chamber master cylinder on it and it fit up easily. Didn't use a proportioning valve, works fine.
Updated my '64's single pot MC to dual using a drum/drum for a '67 Nova. If you don't have front discs you don't need a portioning valve
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