I put on a new hydraulic pump( because the old one was only getting 1500psi). Upon starting the tractor for the first time, I had no hydraulic pressure, 0psi. I loosened the cap, #21, and the valve #25 must-ve opened cuz I immediately got my 2500psi. I tightened #21 and afterwhile I lost pressure. I believe that valve is supposed to open to supply oil to pump. I was going to take the control valve assembly off and look for bad o-rings. What else could be going on?