no high pressure rv2 help!

:wack:new rebuilt compressor using r-12/ new drier . clutch cycles/ max 50lbs high side pressure, Low side pressure is 45+50lbs drops down to 20lbs when clutch is engauged and then as soon as it gets to 20 clutch dis=engauges.the high also drops down when compressor is engauged. when i jump the temperature sending switch inside the clutch stays engauged low side pressure is 15 but i only have 50lbs on high side . temperature inside is 25 degress. system is not low on freon '