This may not be the easy way, but here's what I did.
I cut a piece of plastic for the valves. The valves are made up of bits and pieces from a tamiya lunch box, and a revell of germany steps peterbilt tow truck. I drilled four holes for the lunch box stock dampers, and installed them. On the outside where the hose would connect, I used the chrome inserts for the rear wheels of the tow truck, trimmed the outside portion to make the same diameter. Next I drilled a hole on top of each valve for the pieces that turn, and used damper spacers for the trucks. I put a washer on top for the wheels that open and close the valves. Screwed in place, and I had my valves. Like I said, probably not the best way, but it worked for me. Now I have to get some tubing to finish the plumbing. Hope this helps, and possibly give you some ideas.
Big John