The Stovebolt Site Forums The Shop Area The HiPo Shop Brake questions: modified '57 3600

I'm chasing gremlins on my latest acquisition, a 1957 3600 that was built with numerous modifications some time ago and then driven very little. The truck has a front frame clip from a 1975 C20 (PS, power disc brakes, etc.). The rear axle is a 14 bolt from the same C20. The master cylinder is a Classic Performance Parts frame mount kit with booster, an inline 2 pound residual pressure valve in the front brake line and an adjustable proportioning valve in the rear brake line. My problem is the brake lights won't turn off after releasing the brakes. I suspect the problem is caused by the brake light pressure switch being mounted inline between the residual pressure valve and the disc brakes, thus being kept on by the residual pressure valve.
Questions:
Is my diagnosis correct?
If so, where should the brake light pressure switch be located? between the master cylinder and residual pressure valve? or relocated to the rear brake line (drums on the rear)?

Rear brake line, or mechanical switch on the pedal would seem the easiest.

Truckbiz,
Thanks for the response. It's appreciated.

It's hard to imagine only 2 PSI keeping a switch turned on. I'd suggest going to a switch with a higher pressure rating, or using a pedal-mounted mechanical switch. You can also power the hot side of the circuit with a wire from the ignition so it's powered off anytime the engine isn't running.
Jerry

I'm not sure if this will help your situation but here's a diagram from Speedway that shows the location that your pressure valves should placed.

Craig, your diagram matches my truck except I have an adjustable valve in the rear line instead of the 10 psi valve. Jerry, your suggestion about finding a different pressure switch sounds like my preferred option.
Thanks for your help.

Just wondering here, have you OHM'd the switch out to see if it's working correctly?

Rear drum brakes need a 10 PSI residual valve in the line ro prevent air from being pulled past the wheel cylinder cups. I'd suggest putting on a pressure switch with around a 25 PSI rating or slightly more, in the line to the rear brakes. Depending on how vigorously the brakes are applied, line pressure can go to several hundred pounds, maybe even a thousand or more. A Rolls Royce Silber Cloud I worked on once had two camshaft-driven hydraulic pumps for the brake system that operated at 3K PSI! The proprietary brake fluid could only be bought at R-R dealers, and it cost $50.00 a quart!
Jerry

Craig,
I have not tried that. Thanks for the suggestion.

Shouldn't a pressure switch be right at the outlet of the master cylinder? That would let the pressure go to zero with brakes released and up to at least the pressure of the lowest RPV when you mash the pedal. That might be the simplest solution if the switch is on a tee in the brake line.
Or maybe I'm not understanding how RPV's work.

That was my thoughts... I have never seen one mounted in the rear brake line. I was just assuming (probably wrong) that it had work prior to this.

A RPV is designed to maintain a slight amount of pressure in the brake lines at all times. On rear drum brakes with strong return springs that retract the shoes when the brake pedal is released, and relatively small diameter wheel cylinder cups, a 10 PSI valve is needed to keep the wheel cylinder cups expanded against the walls of the cylinder to avoid air getting past the cups and a spongy pedal developing. Since the only thing that tries to retract a disc brake caliper piston is the distortion of the square O ring as the piston pushes against the pad, and the large diameter of the piston, a 2 pound valve is all that can be used without causing the disc brake to drag. Most of the hydraulic brake light switches I've seen were either located at the master cylinder outlet fitting on single-line brake systems, or in the rear brake line like 1950's MOPAR vehicles. Virtually all auto manufacturers went to pedal-mounted brake switches in the late 1950's or early 60's, mostly because of the proximity to the steering column, and the necessity to incorporate a discriminator circuit to the brake lights on vehicles factory equipped with turn signals.
Jerry

It just dawned on me that I didn't let everyone know what finally fixed my problem with the brake lights. The solution was simple: I replaced the brake light pressure switch. Apparently the old switch had gone bad and wasn't shutting off completely when the brakes were released. Thanks to everyone for the help.

Sometimes we make the BIGGEST problems out of the smallest solutions. My Auto Shop teacher was a firm believer in the " KISS" method (Keep It Simple Stupid), from time to time I have to relearn that. Glad you worked it out.