Jim, if you want to verify the pressure yourself, you can make a test setup like the Chevy dealer or a well equipped shop would use.
You need to build a manifold with a gauge and shutoff valve that mounts in line in the high pressure line, connecting either at the pump outlet or at the rack inlet. It may require an extra hose too so that you can hook it up.
So when connected for a test, it would be:
Pump outlet, tee with a pressure gauge (0-2000 psi), shutoff valve (usually a needle valve, but you could use a ball valve), then the rack high pressure inlet.
You could connect this at either end of the high pressure hose, with the manifold connecting directly either to the pump or rack - or you could use a hose on both sides of the manifold.
To test, close the valve slowly and watch for pressure to build. I don't know what pressure your pump is supposed to make, but it's in the 1000-1500 psi range. Do NOT let the engine run long making high pressure, since the oil will heat up rapidly. Just run it long enough to know that it does or doesn't make pressure.
If you are not making pressure with this test, you probably have a bad pressure relief valve. It is built into the outlet fitting of the pump and you can replace it cheaply and easily without taking the whole pump apart or even taking it off the engine. (We already know you have flow, so it's probably not the pump itself if you have no pressure.) If you have pressure but it's low, then it could either be the valve or pump.
If you are making pressure and are still having problems, you probably have a problem with the hydraulic valve in the rack. Unlike the pump, this valve is not really serviceable without total rack disassembly and you can't buy an over the counter replacement. So if that's the problem, your best bet is an exchange rack.