When installing new ac high pressure switch o ring blows from pressure release. how do i fix this problems?

I just tried to replace my high pressure switch and I overtightened the new one... what a pain... so the whole A/C blew. Nevertheless, I think I understand your question. When you're loosening the switch you will notice a little leak through about two to three turns of the wrench. This is normal. Then remove the switch, replace the o ring, and put the switch back on. You may want to check your manual for specs on how tight to tighten the switch. Like I said, I overtightened mine and the threads separated from the switch and I leaked my whole A/C system dry.