Tractor Repair: Hydraulic problems, hydraulic problems, hydraulic motor

Question
I am have a problem with a hydraulic system and looking for some in-sight into what the problem may be.  The hydraulic system is actually on a equine treadmill.  The belt of the treadmill is driven by a hydraulic motor.  The treadmill was manufactured by Precision Metal Works (Washington state area).  The problem I am having is that the belt on the treadmill does not have enough power or speed to move when a horse is placed on the treadmill.  I have replaced the hydraulic pump, hydraulic motor and have had the electric motor that drives the pump rebuilt.  Without a horse on the treadmill, the belt will run but the not at a very fast speed.  When you put a horse on the treadmill and engage the motor to start the belt turning, there is not enough power through the motor to cause the belt to turn.  I know this is not tractor related but is there anything insight you can provide as to what might be occurring.  The other thing to note is that there is a control dial to control the speed of the belt.  With nothing on the treadmill, you can turn the dial all the way up but the speed of the belt does not change.  Any help would be much appreciated.

Answer
WOW is this a teat or what. Does it have a control valve on this thing in between the pump and motor because there might be a relief valve that is sticking not letting pressure build. Also does the oil get hot? Send me pictures of the unit and I just might be able to help. You have my interest.