Wilwood Master Cylinder Installation Question

[chuckji]

I currently have a vacuum boost disc/drum brake system with a Wilwood adjustable proportioning valve. I think it’s a basic Wild Horses kit. Something like this: https://www.wildhorses4x4.com/product/WH-Power-Brake-Kit/Bronco_Power_Brakes#pdf

I have a new Wilwood master cylinder I want to install, replacing the m/c that came in the kit. The m/c instructions says to use a 10 lb. residual pressure valve for the drum brakes.

I searched the site and don’t see any posts about using a residual valve. Do I need to use one, or does my Wilwood adjustable proportioning valve take care of that? This is the pro-valve I’m using: http://wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-11179

I don’t see anything resembling a residual valve in my current set up, but it seems that it may be built in the m/c...?

Thanks for your help.

 

[Broncobowsher]

In OEM applications they are typically built into the master cylinder. I've read many times they are only really needed when mounting the master cylinder lower then the brakes (old cars that are under floor mount) to prevent gravity drain back. I don't know of any racer who uses them either.

But some people will still want them, so they are still available. Quick source https://www.speedwaymotors.com/shop/residual-valves~8-39207?suppressautocorrect=false
Personally, I would leave it out unless there was some need for it.

 

[Apogee]

The ~10 psi residual pressure valves were typically mounted in the MC outlet ports, and serves to maintain a small amount of pressure on the cup seals inside the wheel cylinders on drum brake setups. If you look at an EB, the 7/8" wheel cylinders have an effective piston area of .6 sq inches, so 10 psi only equates to about 6 pounds of force. This isn't enough to effect the drum brake actuation, but is enough to maintain enough pressure on the cup seals such that they remain "inflated" and in contact with the bore upon brake release, when the retraction spring collapses the wheel cylinders. Without this, the seals can crease and/or fold and leak out brake fluid and/or allow air into the system.

Other applications use mechanical seal expanders/reinforcers and it's possible that modern seal designs don't require the residual pressure to function properly, but that's the history of them.

As noted above, 2 PSI residual pressure valves are commonly used as a check valve in disc brake systems where the calipers may be higher than the MC. Due to the larger effective piston areas of calipers versus drum brake wheel cylinders, 10 PSI tends to cause a lot of drag in disc applications. For instance, the EB/F150 front calipers have 2.875" pistons with 6.49 sq inches of piston area, which would result in about 65 pounds of clamping force at 10 PSI.

Tobin

 

[chuckji]

Thanks for your replies Broncobowsher and Apogee. I got the M/C installed and brakes bled. Working fine so far.