If you even touch a schrader valve, check for leaks
afterwards. Those things are notorious for leaking after they have been disturbed.
Also, low system pressure is as likely to cause a safety
device to shut down a compressor
as high system pressure is.
Knowingly venting refrigerant to the atmosphere is a cardinal sin in the refrigeration
industry. You are required by law to recover the old refrigerant with proper equipment
. That is the big reason why they now require licenses to purchase
some types of refrigerants. They are trying to prevent DIY people without recovery equipment
from venting old systems before recharging.