Yes, the coolant lines to the firewall have O rings. Volvospeed Bay13 has a writeup, or Mathewsvolvo if you can navigate it.

The 850 has 7.5 quarts of coolant. When you pull off the bottom hose, you will drain everything except the heater core. Unless you change coolant type, I would use Volvo brand, no flush is needed. There will be enough new coolant to keep the engine happy.

An inch is a lot. It could be a very slight leak at the heater core or even the cap. A pressure test might be needed to rule out the cap. Also check the bottom of the overflow bottle for residue - wipe it clean now and check it again after a hot run and cool down.

Remember, when replacing hoses, make sure the clamps are in the same position on the plastic fittings at the radiator. And don't make them too tight, you do not want to stress the fittings.

Klaus
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Always willing to listen, just not able to take direction.