A/C leak?

I added refrigerant about 5 years ago and the system has worked well until now. System started not cooling while at idle I decided due to age it was time to replace the receiver dryer/accumulator no leaks were evident i.E. Oil stains. I had some difficulty installing R.D. Just had to separate springs at hose joints. I oiled the o rings and slid joints together. After evacuation of system, I thought I was able to sustain zero neg pressure however the system stopped cooling after two days and was empty after five days. Had pressurized the system with air compressor and was unable to find any air bubbles especially at the receiver dryer. I checked several times! I checked vacuum leaks and would not hold vacuum for three minutes. The system holds air pressure with very slow lose. My next step is to use ultraviolet A/C leak check with light and glasses.
one of my questions is using ac stop leak on the system, does the stop leak endanger the compressor and plug other parts. Also is there a problem with the China made receiver dyers? I had a very slow leak prior to the replacement of the R.D, now I have a significant one. As I stated I looked closely at the new part since there was such a change.
Through all this I went through five sets of loaner manifold gauges and two vacuum pumps O'Rielly's auto parts were all leaking so I purchased a new set and new vacuum pump. Fixed my equipment issues but still have the system issues. Advice about the stop leak would be nice.

Randy Woodby