Originally Posted by mrnicejetta
If you were replacing R12 with R134 I could see that, but why in this situation?
Other than that I agree with most people that have posted.
You do it for something like to make sure its not leaking anymore. If it holds vacum for 30 minutes then its not leaking, if doesnt hold vacum you got a leak. You also do it on any system were you took the a/c apart to remove any moisture that got into the system while it was open. I really dont see the need though if hes just replacing the sensor.
They usually just screw onto the filter/dryer and there is a schrader valve it pushes on it.