If your pressure guage says you have only 20 # in the system, then you don't have enough refrigerant in the system. At 70 degrees outside F12 is around 70 #, R134 is higher. Your system sounds like it lost the refrigerant until the pressure was no longer high enough to force its way out. Did you evacuate the system first, you really have to in order to get rid of moisture and air. You should be able to draw the system down to about -28 inches and then it should hold itself until you begin to add refrig.
Shut off the vacuum pump after about 10 minutes and see if the guage pressure rises, which would indicate a leak. Good luck.