I bought one of these kits:
http://cgi.ebay.com/ebaymotors/77-78-84-LTD-LTD-II-York-A-C-Compressor-Rebuild-Kit_W0QQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247QQcategoryZ33543QQihZ008QQitemZ180089836189QQrdZ1QQsspagenameZWD1V
awhile ago. Yorks are fairly universal in regards to the gaskets/seals/valve plates. Just make sure it's not one of the mini ones.
Replacing the valve plate assembly makes them pump air like new again.