580e weak in the rear

I have a 580 super e. It can barely dig a hole in dirt with the backhoe. If I hit small roots or try to pick up the back of the machine it stops moving and hisses, like the hydrolic oil is by-passing. It doesn't matter if I use the boom,dipper, bucket, ect. It is very weak. The front of the machine digs like a bear. I had a mechanic replace the hydrolic pump, and he checked the pressure at a connection on the passseger side by the loader handle. He said the pressure was fine. The front has more pressure than the back which is normal. He thought the oil was no good cause it appeared to separate in the drain pan. Put in new oil and filter, same thing, weak in the rear. I had an old ck530 that would tear itself apart with the hoe. This can't be normal. Is there something wrong in the spool valves?