Once it does finally prime to 40 or so psi, will it hold it there for very long once the key is off?
Very long being a few minutes before it goes down to 0 again.
If it drops quickly back down, prime it then use a pair of vicegrips to pinch off the return rubber line back above the rear axle, and then see if the pressure continues to drop. This will tell you if the regulator is loosing pressure. (pinch the hose just tight enough to cut off the flow, being careful not to pinch the hose enough to rupture it.
If pressure still drops, then pinch off the supply line with another pair of vgrips with the return still pinched...if pressure still drops quickly then its an injector problem, possibly stuck open.
If the pressure quits dropping once supply is pinched shut, then that an indication of the pump not holding pressure, or the check valve in the pump is bad (causing the pump to basically work double time to accomplish the same pressure), or a split hose inside the tank as mentioned above.
If your going to drop the tank to see if its a hose, you should just go ahead and change the pump and pickup sock if you don't know how old it is. Always double inspect the little rubber hose from the pump to the steel line...if the isolator has been removed (which is required for aftermarket pumps) then you have to cut the steel tube, making a sharp edge on it that splits the rubber hose during install very easy, so just be aware of that.
