Alco RS1 air compressor problem

One of our RS1's -- 1951 build with Westinghouse air brakes --- is popping off the safety at 150 lbs pressure, when it has up to now happily cycled between
120 and 130 pounds. The overpressure blew out the old, brittle gasket in the governor, which we replaced. The governor does not seem to be the problem --- it is clean inside and all its few, simple parts seem intact and in adjustment, the air passages are clear. Which brings us to the unloaders and the input valves, of which there are two on each of the three cylinders. I'm assuming, and asking if anyone knows if it's correct, that a failure of any one of these 6 would lead to air reaching the main tank. Troubleshooting this looks like a big job...the valves are not horribly inaccessable, but if anyone has any suggestions on how to track the problem down without removing all this hardware, we'd really appreciate hearing about it. The Alco manual has some detail but no diagrams and very little troubleshooting info....thanks for any help!

W. B. Vaughan
Gold Coast Railroad Museum
Miami, FL USA

Update on the problem --- we removed the caps and pistons from the unloaders, finding them to be moving freely, except for one cylinder which is throwing some oil, making the unloader kind of sticky. It, however, moves easily enough that we do not think it is stuck open. Air is moving in and out of both air filters, indicating a major malfunction in more than one unloader or we've blown out the inlet valves...which apparently would be a rare occurance. And hard to repair, as the valve bodies covering the inlet valves are torqued 300 ft. lbs and have been sitting there for about 60 years....
We got onto Wabtec's service center in South Carolina, and they have been more than helpful.
With the kind generosity of Wabtec we are replacing all three cylinder head assemblies, which include the unloader and inlet valve assemblies, and will see if that cures the problem. The Westinghouse NS16 governor will need re-calibrating, which we will undertake when we can get the compressor to load.

W.B. Vaughan
Gold Coast Railroad Museum
Miami, FL