2008 Dodge Caravan refridgerant leak

R134 leaking from somewhere that I cannot locate. I have had the dash partially dissembled with no luck finding the leak. It will sometimes hold pressure for a day or two, and cool normally (approx 25 degree drop from ambient). Low side pressure w/compressor running in the blue area on Inter Dynamics can-top gauge. Then there will be the sound of gas venting from under the dash (evaporator, I guess) and low pressure cut-off engages until coolant is added. When this occurs, no obvious odour, 'smoke', or sign of leak detector dye anywhere. I don't know if this is normal expansion valve/orifice sound from refrigerant changing state. No luck with various leak sealers. I have been careful not to overcharge, and the electric cooling fan is running normally. Engine temp is in center of normal range.
Is there a high pressure relief valve that may be activating, perhaps at too low a pressure?
I cannot find a manual for the 2008 year yet, at least not one I'm willing to pay high dollar for!