I took a jacket to the dry cleaner today, and in an inside pocket I found a diagram provided to me by Forest Goodhart at least a year, year and a half ago, after a heated debate regarding the function and design of the brake system shuttle valve. I had made the contention that the shuttle valve's primary fuction was to block off the flow of fluid to a compromised system, but Forest educated me. As it turns out, while it may have this effect to a certain degree, it's strictly a secondary result. The primary purpose is simply to illuminate the "you are about to crash" light on the dashboard. Last week, we were talking brakes, and specifically talking about a brake system that had suffered a degradation in performance for no known reason. I speculated that the shuttle valve may have become stuck in a displaced position. Realizing that this apparatus is somewhat difficult to visualize, tonight I took advantage of a rare night at home (!) and scanned Forest's drawing.
Click to view image
This drawing shows a typical (i.e. non-Pantera) valve assembly. The Pantera unit is philosophically the same, but boasts an extra port into which the brake light switch is installed. For the purposes of our discussion, however, this diagram does an excellent job of showing how the system works. On the right side, the system is shown in a normal configuration. Brake pressure to one system (say, the front) comes in at the bottom left and goes out the top left. Pressure to the opposite system (which, in this example, would be the rear) boes in at the bottom right and out the top right. When all is good, the valve in between remains centered. The left drawing shows what happens when one system is compromised. In this drawing, the left-hand system has suffered a leak, and the differential pressure has caused the piston to be displaced to the left. The stepped body of the piston compresses a plunger in the "you are about to crash" switch, which completes the circuit and illuminates the corresponding light. When this switch becomes gunked up, bleeding the brakes can cause it to stick in this extreme position, permanently compromising braking performance to that system. The solution is to remove it from the car, remove the end cap, stick compressed air into the far side, cover the exit port with your thumb and fire away, being careful to aim the thing so the piston doesn't come flying out and go into low earth orbit. Once it's out, soak the valve and the piston in brake cleaner, scrub them clean, use some brake assembly lube and carefully put it back together. Or, do what I did--rig up a substitute system and do away with the shuttle valve entirely:
This photo also shows the typical installation of an aftermarket master cylinder with billet aluminum adapter, and the Wilwood 2 psi check valves which keep nominal pressure on the calipers and prevent pad migration (but also create hellish juddering at the steering wheel if the rotors become warped!)