a/c high pressure on low side

[mustangd]

hello-
just brought home a 2003 cvpi with 78k. a/c blowing warm air, compressor not cycling. the car had been setting since taken out of service, so thought i'd try putting some of the shaving can a/c refrigerant/oil in, as i have done this in the past and had some luck. ok, 2003 is a little different from my 2001, but, high and low sides on the a/c have different size fittings, was on the fat tube with the smaller fitting, when i tried to hook up the the hose, high pressure escaped, so much that i couldn't attached the hose if i wanted to.
thats a new one on me, high pressure on the low side.
any info on possible troubleshooting this would be much appreciated, always try to explore repairing things myself before a repair, hate it when i pay dearly for something that was simple.
thanks!