I had a leak in my 4 ton split system. The leak was found on a coupler joint. So the service tech pumped down the system and cut out the problem joint and brazed in a new joint. The lineset was pressurized with nitrogen to 150 psi and left sitting for about an hour. Then started vacuum procedure where it took about 45 minutes to pull down to 500 microns. The vacuum valve was closed then the vacuum pump was turned off. After the pump was turned off, the pressure continued to drop until the micron guage read 240 microns. This process took about 8 or 10 minutes. This seemed strange to me, I would have expected the pressure to rise after the pump was turned off. The lineset is about 60 feet long with 3/8 and 1 1/8. Does this delayed reaction sound like a problem to you guys or could it be the length of the lineset that causes this.
source https://hvac-talk.com/vbb/threads/2228815-Vacuum-Pressure-Keeps-droping-when-pump-is-turned-off?goto=newpost
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