What year/spec 727?

The line pressure varies with throttle pressure input so without the throttle pressure (kickdown) linkage hooked up it'll check low, although 30 is really low. According to the factory serv manual the minimum should be 55 psi with the throttle pressure lever forward. With the throttle pressure lever all the way back it should be 90-95

The line pressure port only measures pressure in forward gears. Thus the zero reading in reverse. You measure the pressure for reverse off the rear servo port.

Check the pressure at the rear servo port in reverse and post the results.