I'm getting off the topic, but as far as I know, there is no pressure reduction in the factory combo valve, so it would be for fine tuning. Might be good with a disc rear (Mazda or Nissan).23yrRebuild wrote:Can a proportioning valve somehow be used to make the rear drums brake more? I'm under the impression these valves reduce pressure, so in order to increase rear braking, it would need to be in the front brake circuit. Any comment?
You're correct, they only reduce pressure. The roadster uses non-servo brakes, so the leading shoe does most of the braking. Servo brakes have a floating adjuster, so when the leading shoe grips, the force also wedges the trailing shoe increasing the brake force. Servo brakes usually need the pressure reduced to prevent lockup. Also, most other cars don't have half the car's weight on the rear wheels and dive more than Roadsters, both of which would need less rear pressure. I had an MG kitcar with a Pinto drivetrain and no proportioning valve. Anything above normal braking would immediately lock the rear wheels. I added a Wilwood valve under the dash, cut the pressure 50% and problem solved. That car had a very light rear and large (for a 1500 pound car) servo rear brakes.
On my 69, the rear wheels will lock if I totally mash the brakes. My front calipers haven't been rebuilt and the pads are older, so I'm sure they aren't 100%, and the rear shoes and cylinders are new, so that may be a factor.
