[steamer]

Cooper as a quick guess I would say it's the piston seal or o'ring. Take the cylinder apart and my bet is you'll find the culprit. If it was a valve seal you'd have to push all the oil out of the cylinder back through the valve to the tank. You're not getting that much resistance so it's a short bypass meaning some where in the cylinder.
Also pretty sure it's the bypass valve that's making all the squealing noise too. That's why the pitch changes with speed, it's the volume of oil going through it that resonates the relief spring and ball!
Rob