Yes, the flywheel o-ring is very close and easy to mix-up. I installed the wrong one this past winter and luckily could tell it wasn't right when I was tightening the screws--too mushy!. MAX sent the two o-rings in one baggie with no markings and I chose incorrectly.
If this is your problem, there would be extra side clearance on the pump rotor and low pressure.
"It's not that I don't like people...I just find I am happier when they're not around"