sounds like you might be a bit low, what are your pressures with the a/c running? a 134a system uses 80% the amount of an equal r12 system. do not however fill the system based on the sight glass as you would an r12 system as you run the very real risk of hydraulic lock.
going off the top of my head, the low side pressure should be about 45psi, and the high side pressure no higher than 250psi as i recall. it has been some time since i dealt with a 134a system.