I had A/C retro-fitted to my Merc Vito about a year ago, made by Eberspacher, on R134A. It was fine for a month or two then when I was sitting in traffic one day there was an almighty sound of escaping gas & a cloud of what I thought was steam (I thought the engine was overheating) -it transpired that it was gas escaping from the A/C in the engine compartment. The guy who fitted it had it back & reckoned it was a dodgy 'O' ring. Two weeks later the same thing happened again. Back it goes. A month later it happens again, only this time about two seconds after starting the engine & it's also accompanied by a GREEN cloud of gas this time as the guy's put leak detection dye in . I had a cursory look & as far as I can see the dye was concentrated around the compressor region. My question is is there a pressure relief valve on these systems as that's just what it sounds like when it lets go -there's an escape of gas & then it stops abruptly & there's just enough gas left in the system to de-fog the screen on winter days. The guy who installed it reckonds there isn't a pressure relief valve & I can't find one.