An 85 gallon tank holds 25 gallons of water. I think you would know if it was leaking 25 gallons on the floor in 8 minutes. Must have something leaking somewhere else as well.
Agreed, I just can't find where the leak is. I've checked toilets all indoor and outdoor faucets. I've bypassed my water softener. I just don't see any other leaks except at the Tank Tee. I'm able to use a gutter downspout diverter into a floor drain, so luckly no mess on the floor. I did install a new flomatic check valve 2 weeks, so I guess that can be faulty as well. Can it be the pressure tank? I've checked the pressure in the tank while it was drained, and its showing 38psi. The only other thing I noticed, is that I didn't feel like that there was a lot of water in the tank when the pump is running. When it reached 60psi, I can still shake the pressure tank. But for comparison, I've never really shaken the tank before, so I don't know if its normal for an 85 gallon pressure tank to be shaken...
Just for clarification, my system is currently setup with the black water line coming in from the ground through the wall. That enters the new flomatic check valve which has the Pressure Gauge and Switch in it. I know I'm leaking at that connection, plus one of the plugs on the check valve. After that I'm leaking at a t-connector going into the pressure tank. Other than that, the pump seems to be running normally, on at 40, off at 60. It just doesn't hold the pressure, either due to the leaks, or faulty check valve, or both...
Regardless, plumber is coming tomorrow to fix the leaks. Hopefully that solves the problem of loss of pressure as well, but I'm not too confident after reading this and other posts...