Cascade chambers

I'm having few issues with a few Cascade chambers mainly on 23 refrigerant couple doing it now reclaim a kilo of 23 system holds 3 kilo hp test with nitro at 325 psi for a week with no pressure drop vac it perfect vac and holds it gas it week later short again uv dye no good against this temp any ideas I pressume something opening up on run when cold or hot there are no stubbs on evaporator only rotolocks on comp receiver can anyone suggest anything would helium mix assist as I believe smaller molecule structure or any other ideas I've had systems hold pressure then leak on the evaporator test port but usually takes three months we have checked other side of Cascade usually 404 those levels good so it's not going in there one is a water cooled condensor one air cooled ta charlie