1990 Other Mercedes Benz Models A/C not strong

Air Conditioning problem
1990 Other Mercedes Benz Models 6 cyl Two Wheel Drive Automatic 129K miles

I recently bought this 300 E and the a/c was not real strong. I retroed it for 124 and made the switch. I have put about 3 cans of the kit in 1 can of the synthetic booster in but still not strong and in high temps takes quite a while to get "cool" but never cold. Are the a/c systems bigger than the average auto and I need more 124 or is there another problem. There are times when the compressor seems to not enagage and restarting the car sometimes fixes that issue. How can I tell if there is a leak or if there is a compressor problem?