I'd suspect a failure in the tube in the rear tire at the base of the valve. If the bike was ridden (by you or the previous owner) while the pressure was very low, the pedaling force might have caused the tire and tube to shift, which often tears part of the base of the valve away from the tube.

A pressure of 85 psi is pretty high for a hybrid bike like your Marin. Tests have shown that comfort, bike handling, and even rolling resistance are often improved by using a pressure somewhat lower than the maximum the tire can take.

Here's a tire pressure chart that can provide guidance based on total weight (you plus the bike), tire width, and riding conditions.