ac not working,have some questions

[mustang51js]

i have been trying to fix my ac and allmost got it fixed,i recharged the system and have 250lbs of pressure which is the max i think.The only way i can get the ac to work is to remove the relay and add a jumper wire.I ordered a low pressure switch and a new high pressure switch to replace.First question is do i need to drain the r34 out to change these, and the next question is i tried to put a jumper wire between on the low pressure switch to bypass it and it shorts out the fuse,which doesnt seem right to me and my manual doesnt say anything about it.This is for a 03 f350 6.0, thanks for any help