I think this diagram is pretty close...
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By blocking off the return downstream of the regulator, your pump should be able to get the pressure up to 60 psi.
I could be wrong here, so someone chime in if I'm way off, but if the pressure reaches 60-psi (and keeps climbing to pump deadhead pressure) then this means the pump is good and your earlier pressure loss was because the regulator was leaking back to the return line. The spring loaded valve isn't sealing.
If it cannot get to 60-psi, then either the pump is damaged, has a voltage issue (which you ruled out) or there is a restriction in the pump discharge line such as a clogged fuel filter or clogged pump inlet.
The best way is to put a shutoff valve on the pump discharge line (disconnect at the fuel filter), energize the pump and see if it climbs. If it does, then there's a restriction downstream. If it doesn't, then the issue is at the pump.
I've read that pressure should not exceed 75-psi, so you'd have to have a gauge on it.
Here's my question, though. If the regulator is leaking, can it only leak (bypass) into the return line? Can the leak also be through the vacuum port back into the intake...causing you to fail the pressure test? Looking through the throttle body, is it really clean in there?
I've never done this, but it seems like one way to check for a leaking regulator is to pull the intake, reconnect the harness, pull the coil wire and energize the pump using the jumper terminal. You'd be able to spot a leak then.
If it were me? I'd find a way to block off the fuel supply at the filter, get a gauge on it and see if you get good pressure. Older TBI versions used a pressure test kit that disassembled the piping there. Maybe a parts store rents those kits, but I'm not sure if the fitting sizes are the same or not.
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