I use a 2 1/2 liquid filled gauge with a 3 ft hose to test and I never leave it on the car. I leave a 1 1/2 gauge on the rail as a quick reference. The fitting I use for the rail is a racreonix. JMHO
Adapter,-4M > -4F, Inline, 1/8 FPT Port (ADF-495000): Gauge Adapter Fittings