I just posted about this a few days ago. A 3/4" pex ball valve from home depot will drop right in and get you by. It will also help diagnose whether the valve is the issue or something else:
I'm posting this in case it helps someone else. When I got my M1151 from gov planet it was listed as "AC compressor doesnt engage." To my surprise the compressor worked fine, but it only blew cold for the rear AC. The front AC blew at ambient temperature, gradually getting hotter as the vehicle...
www.steelsoldiers.com
The only thing that weird to me is that it sounds like your AC does work after a few seconds, after that its fine? If thats the case you may have nothing wrong at all. In my case the front AC didnt blow cold after any amount of time, in fact it just got hotter over time before I put the valve in.