How hard is it to add an A/C pressure switch to a car that doesnt have one?

[WH23G3G]

What is involved with adding a pressure switch to an aftermarket A/C system on a car that didn't originally have a pressure switch worked into the A/C system? I'm switching to 134A so everything is being changed except the evaporator which was professionally repaired and flushed. Where can you find those flat nuts like used on the radio knobs, they hold the aftermarket A/C controls to the dash. I can't find them at Ace hardware or Lowes. 

[defiance]

For the pressure switch, it should be extremely easy.  The switches are pretty cheap (<$15), and any good a/c shop should be able to make an extension fitting with a screw-in port on it for the switch (probably cost you $25 or something to make it).
The knobs, though, I dunno

[WH23G3G]

I'll probably have the shop install it. Is it absolutely necessary to have a pressure switch?

[defiance]

Absolutely.  The low pressure switch actually protects against overpressurization, too.  Without a low pressure switch, the compressor could continue to run even after pulling near-vacuum in the low side hoses, thus continuing to build pressure in the high-side pipes.  The best that might happen would be wearing out your compressor quickly.  The worst could be bursting a high side line at extremely high pressures...

Definitely gotta have a low pressure cutoff switch

Although some systems actually use a thermostatic switch as a substitute, I've never actually dealt with those systems directly, so if there is a thermostatic switch in the system anywhere (it would probably be around the evaporator, I believe) the above might not apply...

Oh, as a note, the best general a/c resource I've seen on the web is www.acsource.net/acforum - a lot of people who work in a/c shops frequent that forum.

[Nacho-RT74]

Usually dryers provides a source for pressure switch. If is not installed then is a bolt on its place to plug the hole.