AC Manifold question

[Notnilc20]

all these videos on Youtube show that after pulling vacuum and then closing the valves at the manifold to see if the system holds a vacuum. However, wouldn't this just close the manifold? I pulled vacuum and then closed the valve at the manifold on both high and low side and left it to sit for 2 days and everything held fine. What concerns me is the guy I bought the car from told me there was a leak and the system is in fact completely empty. Also, I removed the hose connectors at the low and high side at the ac lines and the vacuum still held. Why didn't this release the vacuum if I'm measuring vacuum at the system. Only when I opened the valves at the manifold did the vacuum go back to zero.

Obviously I'm missing something here because everyone says to do this step and see if vacuum holds. Please help. Thank you.

[JohnHere]

all these videos on Youtube show that after pulling vacuum and then closing the valves at the manifold to see if the system holds a vacuum. However, wouldn't this just close the manifold?

Closing both hand-wheels on the Manifold Gauge Set (MGS) shuts off the MGS center port to which the yellow hose is connected but does not stop the blue and red hoses from registering system pressure (or vacuum) on the respective gauges as long as the adapters are connected to the system ports and the adapters' thumb-wheels are turned to the open position (fully clockwise).

I pulled vacuum and then closed the valve at the manifold on both high and low side and left it to sit for 2 days and everything held fine.

That is the correct procedure.

What concerns me is the guy I bought the car from told me there was a leak and the system is in fact completely empty.

Unless you registered at least some pressure on the MGS when you first hooked it up or depressed one of the Schrader valves to test for pressure, there's no way to tell whether the system had any refrigerant in it when you purchased the car since you've already evacuated it. Also, the system might hold vacuum but leak under pressure, and vice-versa.

Also, I removed the hose connectors at the low and high side at the ac lines and the vacuum still held. Why didn't this release the vacuum if I'm measuring vacuum at the system. Only when I opened the valves at the manifold did the vacuum go back to zero.

When you removed the adapters at the system's ports, you would still register vacuum on the gauges because the red and blue hoses remain sealed until you crack one or both hand-wheels on the MGS, which would then open the MGS to the atmosphere via the center port.

[Notnilc20]

Ok, so the hand wheels at the end of the hoses open and close along with depress the valve on the ac ports on the ac lines. That makes sense. Thanks Johnhere.

[JohnHere]

Ok, so the hand wheels at the end of the hoses open and close along with depress the valve on the ac ports on the ac lines. That makes sense.

Yes and no. The adapters at the ends of the red and blue MGS hoses open and close like regular valves--and either depress or release the Schrader valves at the same time--only when they're attached to the vehicle's high-side and low-side quick-connect service ports. If the adapters are not connected to the service ports, then the valves remain tightly closed regardless of which position the thumb-wheels are in.

That's why you would need to crack the MGS hand-wheels to release any vacuum or pressure still remaining in the hoses and MGS itself through the MGS center port (without the yellow hose attached to it). Of course, you can accomplish the same result by loosening one of the hose fittings at the MGS, allowing the pressure or vacuum to bleed off.