854x4

My a/c quit blowing cold 50 degree vent and now only blows cool 70 degree vent. So I hooked up my gauges and I'm getting 40 on the low and 250 on the high, when I rev it up to 2,000 the low goes to 10-20 and the high goes to 300-330 all the lines are hot except the one from the compressor low side to the expansion valve. Also my expansion valve use to make a hissing noise and now it doesnt. Now heres the part I dont understand when I shut off the a/c the pressures dont equalize the low goes to 90 and the high goes to 250. I think the expansion valve maybe bad or there is a restriction somewhere in the system. Also I converted the system to freeze 12 last year and it worked fine for the whole summer and up untill last week. Any Thoughts?