To do fuel rail and injector pressure test disconnect the fuel line at the outflow side of the filter and also disconnect the fuel line return near the filter. Get a tee connector connect the inflow and return fuel lines to it to make a closed loop. Connect pressure gage with a shut off valve and the air pump to the 3rd outlet. Need shut off valve to prevent air bleed back thru the air compressor.
Pump up to 50 psi shut off valve and observe the gage. If all ok gage should remain rock steady needle reading for hours. If not there is a leak. Do with cool fuel.