The hoses in our HVAC system do allow permeation, though it is (should be) very minimal at this point in the car's lifespan. Certainly the newer barrier hoses are far superior to the old hoses on R12 systems, that's for sure.
Also, as touched on by others, don't use the R134a canisters that have additives. Those "stop leak" type additives won't help except in very certain applications (I believe one is a standalone product from Cliplight). Bog-standard basic refrigerant is the best choice, and the most economical.