I'll bet the manufacturer of that cheap guage didn't put as much thought into it as you have. And your logic has no bearing on the construction of that guage. Why did you reopen the valve and walk away?? Like the previous poster said you need to have that system evacuated before ya blow something and it gets expensive. Your system needs to be evacuated,leak checked,repaired properly and then have the proper refrigerant charge and oil weighed in and recharged. These "A/C Pro" kits are gonna be a real money maker for alot of us guys.