They are replaceable- the top half screws off- this site sells them I believe-- but I am here to tell you that the valve leeking is not your issue- a good plastic cap with a good rubber element (washer) inside will seal off that leaky ball valve- my leak detector confirms that fact many times--
Your issue is, if and only if you have a blue labeled compressor (factory OEM unit), a belly leaker- look and see if you have oily greasy mess around the belly of the compressor- leaks at the head interface to the cylinders are guaranteed if yours is an OEM delphi Ht-6 high mount unit . been there, done that numerous times-- I reseal them- take compressor all apart, clean all sealing surfaces, and put in new o-rings and new shaft seals-all obtained off the Bay....
'95 Chevy 3500 454 Dually 142,000 miles
'94 Chevy Astro Van 358,000 miles!
'86 Buick Park Ave 192,000 miles
'86 GMC S-15 Pick-up 150,000 miles
'92 Chevy Lumina Van - 265,000 miles- given to me to see how far it can go! all above are white!