Close the black handle. Prime from the blue valve. Let air out the hex plug. When all air is out, replace the hex plug, close the blue valve, open black valve and have some water on, then turn on pump.
But it really sounds like the jet or venturi nozzle is plugged with something since it only builds to 20 PSI. Remove the plug under the suction pipe, and use a wire to clean out the jet nozzle.