I just read this article:


"For drum brakes to function correctly there needs to be a 10 lb residual pressure valve in the system. This residual valve will maintain a 10 lb pressure on the drum brakes to counter the spring presure in the system. This keeps the shoes out close to the drum giving a higher firmer pedal. Without the residual valve in the drum system, every time you step on the pedal all the fluid goes to moving out the shoes before drum contact.

Is this correct for our first gen Camaros?

There should be a residual valve in the master for drum brakes. It is in the port(s) leading to the front and/or rear drums.

I have never seen one hold enough pressure to overcome spring tension though. They're only about 10psi. You can watch the shoes move in and out with someone working the pedal and the drums off, and it's pretty obvious that this is a myth. Open the bleeder and see a small spurt of residual fluid, but the shoes do not move at all. Obviously 10psi does not hold the shoes out. It takes about 50-100psi. This is a test one can easily perform for oneself to see.

Residual valves are there to keep the wheel cylinder cup seals from allowing air to be sucked in when the master cylinder pistons return. The master pistons return faster than the w/c pistons, and it creates a suction. The wheel cylinder seals are somewhat conical and need residual pressure to hold the seal against the bore. If they allowed negative pressure they would suck in air.
Modern seal compounds may have overcome this problem.


They are also used when the master is below the level of the slaves (caliper/wheel cyl) to prevent fluid drainback. Some people use them with low-drag calipers instead of.... nevermind. they just use them.

I am curious about this because I bought a rebuilt master cylinder. Can I see the residual pressure valve with the brake line off?

It's behind the brass outlet port(s) in the master. I dont know if you'll be able to see it or not. You could try poking a paperclip in the port and see if you feel spring tension. Not sure if this would work or not.

There should be one in the rear brake port for a disc/drum system, and one in each port for a drum/drum.

The spring is pretty weak, but you might feel the valve. Here is a page with a pic of what you are looking for:
http://imperialclub.com/Repair/Lit/Master/232/Page04.jpg