Continuation of a previous post about vane pumps.Tractor came with a 17GPM vane pump, factory flow divider, with a spool valve on the priority side of the flow divider, factory 4 spool on the ex side. I have no flow rater, but am using the digital tach on the air seeder fan to establish flow rates.

Flow ( fan RPM) maxes out at about 1700 RPM,  and drops slightly after that.  hydraulics get noisy above 1800 ish RPM.  I have eliminated any possible places where the flow may be escaping.
- Ran with relief valve tank hose disconnected, no flow at full RPM, so not going over relief.  
- Priority valve load sensing relief valve, no flow while disconnected.  
- Steering return line is well under the flow of 1 to 2 liters per minute in the manual, at full throttle, full load(fan)
- When the flow control is set maximum either way, there is no flow to the other control valve/set of valves. 

Tested with flow control and directional control valves out of the circuit, just pump, relief, priority then right to the fan, return to tank.  I can achieve considerably higher RPM, at lower engine RPM, but still noisy at full throttle.  

Bypassing the flow control, or directional control valves both increase fan RPM, but less than the previous test, hydraulic still noisy at high RPM.

Since oil isn't bypassing anywhere else, it is apparently at the pump.  Pump is in good shape, I simplified the inlet as much as possible, inch and a half hose, 2 feet long, inlet a few inches above the oil level in reservoir.  

So how is the flow dropping off more with higher load( more back pressure from the valves)?  If it was a problem on the suction side it shouldn't matter how much restriction on the pressure side should it? Flow control is 3/4 inch, control valve is 3/4 in and out, but 1/2 inch ports, all hoses are 3/4 including the couplers.  

Running the pump with the pressure hose directly back into the tank makes no noise, but I couldn't test flow.

Oil doesn't appear aerated, not bubbles or foam.

It was much worse, resealed the pump, modified/shortened the suction side, new filter.  I just removed the suction screen, contaminated, but not enough to restrict much IMO.

Relief is set very high, 2800 ish.

Local hyd. shop says this pump is good to 3000 RPM, accessory drive is 1.1 times engine RPM, and running above the reservoir won't affect it. 

Internet claims cummins 855 accessory drive is  either 1.33 or 1.5 times engine RPM, and Vickers literature shows max pump RPM of 1800, tractor goes to 2100RPM, accessory drive going faster by?  So is the RPM my issue?

I don't need to run full RPM, but do need more flow.  Flow was more than adequate with all valves bypasses.