Question: I recently installed a front disc brake conversion on my 1942 Chevy truck along with a new master cylinder and lines. Now the brakes require me to pump them several times before they work, what gives?
– Mark in Ontario, CA
Answer: Mark, Thanks for reaching out to Chevs Of The 40s with your issue. Like your truck, many cars built as late as the early 1950s had the brake master cylinder under the driver’s side floor. This certainly worked for the factory components, but when you modernize, sometimes you end up missing something along the way. In this case, it sounds like your brake system needs a couple of residual valves between the calipers/wheel cylinders and the master cylinder under the floor.
The issue at hand is that unlike a newer vehicle where the master cylinder sits high on the firewall, your MC is under the floor, and level with or below the calipers and wheel cylinders. When you let off the brake pedal, the calipers/wheel cylinders push fluid back to the MC, so they can retract. When the MC is below the brake assembly, the fluid flows all the way back to the master, leaving you with brakes that have fully retracted, the pads/shoes are too far away from the rotor or drum. This requires you to pump the pedal to get the brakes pushed back to position so you can stop. Not only is this a hassle, it is also incredibly dangerous.
The solution to your problem is a set of residual valves. These are check valves that allow most of the brake fluid to bleed back to the master cylinder, while maintaining a minimum amount of pressure on the caliper or wheel cylinder so they don’t retract too far. For disc brakes, you need 2 psi, and 10 psi for drum brakes. Master cylinders that are designed for under-floor use typically have these built-in, but when you piece something together or use an aftermarket MC, the residual valves are required for an under-floor master cylinder.
Learn more and shop now at chevsofthe40s.com!
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