What you'll need to do is think of two separate units. The split reservoir (closest to the drivers side fender) controls braking (front reservoir for rear brakes and rear reservoir for front brakes) and the large reservoir (farthest away from the drivers side fender) is for power assist. A soft pedal may be indicative of a leak in the braking system. Start by checking the wheel cylinders (at the rear) and then the calipers (frony disc for hydraulic leaks). Then work you way up to the combination valve and finally the master cylinder. IMHO, I would empty the powermaster motor larger reservoir to test if you want to continue to test with the ignition off and the protective cap removed. Have you replaced the master cylinder and are now encountering a low pedal? If so, did you bench bleed the mastercylinder?
Now, regarding the powermaster side of the mastercylinder. It needs fluid to cover the suction port (small port in the front on the floor of the reservoir) at all times. The rear port (on the floor of the reservoir) is the return port. The powermaster motor pressurizes the the accumulator and the pressure switch turns the motor on and off at specific pressures. The accumulator is split into two diaphrams, with one side holding pressurized nitrogen. As the brake fluid acts on the diaphram, it creates the pressure reserves for power assist braking. When the system is pressurized and you push the pedal, the pressurized diaphram in the accumulator pushes back to give you power assisted braking. The fluid is retured through the return port and cycled back through the system. This is the reason that when you press on the brake pedal (with the fluid cover removed) that fluid shoots out the top, the fluid is returning via some pressure. If you get air in this part of the system, the motor will run constantly. DON'T let it run for more than 20 seconds. You could test the power assist side, but it requires brake pressure gauges and you'll need to make an adapter. I don't think you're having problems with the motor though. I think you've encountered air in the brake system. Maybe the mastercylinder is leaking past master cylinder piston. The master cylinder rebuild kit is still available from GM and is quite an easy job to do. You won't have to hone the bores, because the bores are anodized and if you see pitting, then the mastercylinder is done for. By the way, DONOT TRY TO REMOVE THE FEED LINE (BELOW THE ACCUMULATOR), THE ACCUMULATOR OR PRESSURE SWITCH WITHOUT DEPRESSURIZING THE POWERMASTER SYSTEM. THAT IS AT LEAST 10 PRESSES OF THE BRAKE PEDAL (WITH THE IGNITION OFF) OF 50 POUNDS OR MORE. THOSE PARTS ARE UNDER EXTREME PRESSURE AFTER THE MOTOR PRESSURIZES THE SYSTEM. OVER 700PSI!!!! GOOD WAY TO DAMAGE THE CAR OR LOSE BODY PARTS!!!!
