I believe that it is now "illegal" to just replenish an air-con system .. it has to have a pressure check for leaks first .. environmental pollution and all that .....
.......
We'd better take all those cans of R134a from the shelves of Halfords if that was the case. IMHO it's illegal to knowingly fill a leaky system with refrigerant.
It is illegal to knowingly refill a leaking system.
As I have said in the past, and many have said in this thread, a proper vac and regas is the only real way forward.
Incidentally, the A/C should be serviced every couple of years by doing this and replacing the receiver / dryer.