If the old compressor never had one and never needed one? what is the exact reason for adding one to this new install?
Back when the old systems used R12, they ran on lower pressures and, had a "blow-off" valve which would release Freon into the atmosphere if the system over-pressurized. That is totally a no-no now! In the heat of the day, with R134a, if you shut off the engine for a while, maybe go eat lunch. It gets heat soaked to the point when you start it back up, it'll spike pressures up over 500 PSI. With the HP Switch, it'll shut off the compressor @430PSI (with the one I have) and turn it back on @200PSI until the system equalizes to normal pressures at ambient temps. I had an HP Switch blow out of the back of the compressor due to that, sounded like a shot gun going off! After cleaning my shorts out, I found the groove for the snap ring broke ruining my almost new R4 compressor.
That's when I started doing research. On the systems without the "Max" button they had a one-wire HP Switch, in the back of the compressor, that would close the recirculation door to help bring down pressures and stop freezing by bringing in warmer air.
So, my little work around is simply protecting the system from grenading itself again and, has worked great here in the desert with almost 120° ambient temps for almost 3 years now. HTH