The Shay brake valve was redone several times to get a satisfactory design. One of the complications was that the particular Shay modeled used steam to apply the brakes and also steam to release the brakes. The Heisler doesn't have that complication since the brake cylinders have spring returns. Recently Dick McCloy showed me a Clippard regulator that had been modified to make it into a brake valve. The modifications are to replace the fine thread adjustment screw with a very course acme thread screw and add a handle to the screw. The course thread makes the output go from zero pressure to full pressure in one turn or so.
The neat thing about the regulator type valve is that the pressure on the brakes is relatively constant independent of the supply pressure. Another neat feature is partial braking on long downgrades.
I've used the Clippard regulators for the oil atomizer and am using one for the blower on the Heisler. Recently Enco had their 3/8-12 acme tap on sale and with a discount for about $40 so I bought one with construstion of a brake valve in mind.
The tap is pictured above. Note that it it cuts the treads in two steps, the first step cuts a V and the second step squares up the V. Dick McCloy had some 3/8-12 threaded rod so I got a piece and went about building a brake valve.
The photo above shows a disassembled regulator. The inside of the next to the last piece on the right (the cap) needs to be rethreaded with the acme threads. The last piece on the right (the adjustment screw) needs to be replaced with the acme threaded rod. The cap is sealed to the body with the blue sealer visible on the cap. I clamped the body in the the lathe 3-jaw chuck and removed the cap using channel lock pliers. The nuts that screw on the top of the cap were screwed on tight and used to hold the cap in the 3-jaw chuck. The tap was put in the tail stock chuck and the cap turned by hand to tap the threads.
A description the regulator operation is on the Burner Construction page.
I haven't used the valve at the track yet but have used it on the test stand and it performs great --- exactly as expected with one exception. The valve Dick McCloy showed me seemed to respond with less turning --- the screw tread is probably 8 TPI or 10 TPI. I also recalled that he said he got the valve from Tom Bee.
So, went to Tom Bee's website (Tom Bee 1.5" Scale Products) and discovered he has a whole line of useful brake parts including the brake valves for $35 with a really nice handle. I could buy all the valves I need for a lifetime for much less than the cost of a coarser thread acme tap so next time I will buy my valve from Tom Bee.