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The hydraulic unloader doesn't have a gasket. The X-ring seals it to the case, and the cup seal holds in the oil pressure to activate the valve. There SHOULD be air coming out of it after it stops, but not while it is running. Check to see if the oil pressure is good. Make sure the cup seal is put on the plunger end of the unloader valve, installed so that it captures oil pressure within the bore that it is installed in. You can check which port is which by putting a copper tube on one port and seeing if air comes out the other, then pull the plunger outward to see if the air shuts off, mimicking the operation of the valve under oil pressure. Did you make sure that you did not pinch any of the valve discs between the valve body halves as you were putting them together? It's easy to do, and will keep valves from sealing. Were the valve seats flat? Were the guides worn? Worn guides can let the discs run off center, sometimes allowing a small sliver of the air passage to remain uncovered, allowing air back throught the valve. Were there any copper sealing washers under each valve in the cylinder head port recesses? Also, when you remove unloading towers, you must back off the threaded collar that they screw onto, so that you can properly retorque them after you put the valve covers back on. They may be loose, allowing the valves to float around.
Make sure that you are measuring interstage pressure and not discharge pressure too, as final discharge pressure should definately be equal to what is in the tank. If it IS interstage pressure, you have high pressure valve problems that need to be fixed. Check each valve by filling it with Varsol (mineral spirits),or very light oil, and checking for leaks. Some leakage is normal, but if it all runs out in a few seconds, you need to true up the valve seats by hand lapping them flat again. Use a flat heavy piece of steel or a thick piece of glass and some wet and dry paper, a sticky backed snading disc, or good old lapping compound to bring them true. Do one at a time so you get them all back together correctly. Valve discs should be reusable if you flip them over. Rusty or pitted or scored discs are probably garbage.
You must replace those missing bolts, otherwise you will never know if air leaking or a blown gasket might be a cause of low output. Sounds like someone had the heads off already. Make sure the air filters are clean. If oil pressure is low, try adjusting the oil pressure regulating nut to increase it. If that doesn't work, you might have to pull an inspection cover off the side of the crankcase and check the counterweight on the crankshaft for the presence of an oil pressure relief valve (it will look like the head of a bolt with a hole in the middle. You can unscrew this valve and see if the spring loaded shuttle is moving properly. If it fails to seal, you will have low or no oil pressure and the compressor will not load up properly.
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