I have a feeling that you have a clutch or fiber disk failure and the screen to the pump is plugged.
Normally you need to do a pressure check/flow test on the PS/hyd system to determine the problem
What type of oil did you use when you changed it?
I once changed oil in a cherry 1370 and the oil I used dissolved the glue underneath the fiber disk plates and plugged up the oil intake screen.
P.S> An 870 is worth fixing so if you do have a major problem please consider fixing it