I have a 740T, 1989, auto. 170K miles. I recently changed the A/C system over to R134a. About four years ago the compressor was replaced with a used rotary, and the dryer replaced, then charged with R12. The R12 had since leaked out, so I made sure there was no pressure in the system, and put in three cans or 134a, without evacuating. The 134a cans were the type that has compatible lubricant in the can. The cooling was weak, so I checked the pressure at the valve on the dryer- low side-, and got 45 lbs. I bled the system down to about 30 lbs at idle, and took it out, to find that it cools less than before- barely cools is more like it. At 30+ mph, the compressor now cycles on /off, but does not at idle, so I gather this means there is not enough 134a in the system to keep the compressor going. Can someone tell me what I need to do now to get my A/C back up to spec?

Thanks