A/C wont kick in
 

i have a 79 K5 and am having a wierd issue,
i replaced all the components in the A/C system put the vacum on it found no leaks .
thats the good news the bad news is the clutch wont kick in , i have jumped the cut off switch with no results checked the wiring in the entire system everything checks out,i put pressure on the system still nothing . at this point i installed 2 other compressors that work fine on my other trucks and still the clutch wont kick in.

HELP any suggestions?