I’m going to challenge your brake residual pressure valve explanation (Your Jeep, May ’18) regarding the small residual pressure in the brake lines of drum brakes. I’m sure we were told in school in my apprentice days 50-plus years ago that pressure was to keep the cylinder cups sufficiently seated so as not to leak. The star wheel adjuster keeps the shoes up close to the drum. The return springs are strong enough to overcome that minimal line pressure. That’s my story and I’m sticking to it.
Love your tech stuff, even as a nitpicker.
Clarke Lacey Vancouver Island, British Columbia
You’re right! While it is true that the drum brake shoes are held in place close to the drum braking surface with the adjuster assemblies, the wheel cylinder pistons and plungers can still retract away from the shoes. The residual pressure valve in a drum brake system is used to keep the wheel cylinder pistons from working their way back into their bores when the brake pedal is released. A small amount of line pressure (around 10 psi) allows the wheel cylinder plungers to maintain contact against the shoes in the drums. This provides much quicker drum brake engagement when the brakes are applied and reduces brake pedal travel.