Master cylinder residual valves

[chewychevy67]

I just bought a disc brake kit for my 67 and I bought a master cylinder a long time ago for drum/drum when I was going that route. Now I just checked and neither side has a residual valve installed for drums when it was rebuilt. I have not used this yet. Can I use this cylinder for my disc/drum set up? Will having no residual valve in the back be an issue? Really not sure why they weren't I stalled in the rebuild. I will be using the disc combination valve.

[chewychevy67]

This is the master cylinder with no residual valves I have.

[B. W.]

You don't need residual valves if the MC is mounted above the wheel cylinders or calipers (think older trucks / cars with MC mounted under the floor)

I would still get a MC for disc/drum. the stock disc/drum mc has a larger bore & a larger reservoir (1.125 vs 1")

[chewychevy67]

Ok. Just curious. Does the larger bore add more pressure or just move more fluid?

[B. W.]

The larger bore does both. the cylinder has more volume & the larger the surface area of piston the more force is applied.

[chewychevy67]

I bought this master cylinder which is supposed to be for disc drum and it too has no residual valve in the rear drum line. My original cylinder did have them installed. Just food for thought

[davepl]

Originally Posted by B. W.

You don't need residual valves if the MC is mounted above the wheel cylinders or calipers (think older trucks / cars with MC mounted under the floor)

I would still get a MC for disc/drum. the stock disc/drum mc has a larger bore & a larger reservoir (1.125 vs 1")

I don't think this is correct, actually.

For drum systems you generally have a 10lb residual valve to hold the drum brake "ready".

For under-floor systems then you need the 2lb residual valve just to prevent drainback.

So, I think the residual valve is still required when running rear drums, but I'd search/ask a little more, as I'm no brake guru!

[chewychevy67]

For some reason all the rebuilt ones here have no residual valves in them. Did rebuilders just stop putting them in?

[B. W.]

I was taught that a 10 psi RPV was needed on drums to prevent air from entering the system when the wheel cylinder cups are retracting (when you take your foot off of the brake) That was a while ago, newer wheel cylinders don't need them, they have cup expanders in them. Because of this, master cylinder manufacturers stopped putting in RPVs (save money?) There may be some MCs with RPVs, I don't even check anymore.

Below is a an excerpt from an ASE test prep guide, explains it better than I can:

"Many master cylinders designed for drum brakes incorporate a residual check valve to keep a slight amount of pressure on the system after the brake pedal has been released. These residual check valves are used on drum brake systems to keep around 5psi. up to 12psi. of pressure on the wheel cylinder's cup seals and resist any momentum of brake fluid returning to the master cylinder. Since the valve is keeping constant pressure on the system, any small leaks will result in fluid leaking out instead of air leaking in. Residual check valves keeps a constant pressure on the wheel cylinder's cup seals. These valves are located inside the master cylinder port leading to the rear drum brakes. They consist of a check valve and spring that allow brake fluid to enter the master cylinder only until a designated pressure is reached. Once this occurs, spring pressure closes the valve maintaining low pressure on the drum brakes. Many vehicle's today use cup expanders in the wheel cylinders that keep an air tight seal. This prevents air contamination eliminating the need for residual check valves."

[chewychevy67]

I took out the valve from my original master cylinder and pulled the brass cap from the rear line on my new cylinder. I will install it and press a new cap on it. If the factory had it then there must be a reason it's there.

[RichardJ]

The last line in B.W.'s quote above tells the story.

>> Many vehicle's today use cup expanders in the wheel cylinders that keep an air tight seal. This prevents air contamination eliminating the need for residual check valves." <<

If you want to reinstall the original Residual valve in your new MC, it won't hurt or change anything. Maybe just a little insurance.

All the aftermarket brake manufacturers sell stand-alone residual Valves. Their main use is intended for HotRods/ old cars where the MC is mounted under the floorboards and therefore lower than the wheel cylinders. In those cases, the cup expanders in the wheel cylinders will not prevent gravity from pulling fluid from the wheel cylinders and back into the MC. That would cause AIR to enter the wheel cylinders, past the cups.

The aftermarket brake manufacturers sell 2lb and 10lb residual Valves. The 2lb Valves are used for the disc calipers. Anything more than that and the caliper piston will not pull back away from the brake rotor and the brakes will drag. Again, only needed for disc brakes, WHEN the calipers are higher than the MC.

The original MC residual Valves, designed for drums, should never be used in the disc brake side of the system.


Up into the mid 1950's there were still cars and trucks with MC's below the floor and the residual valves were needed and the best location was inside the MC. They continued to use the residual valves for safety as the MC's were moved up on the fire and until the wheel cylinder cup spreaders were improved to the point they were more reliable.