Dear Carl,
Just a small preamble about power transfer with hydraulic circuits: power is the product of flow and pressure. In an ideal hydraulic system,
- High flow and low pressure difference = low work (power).
- High flow and high pressure = high work.
- No flow and high pressure = no work being done.
There are lots of hydraulic calculators on the web that let you interactively explore the tradeoff in exact detail, but remember that this assumes no wear or losses in your pumps/motor (which may be an issue).
If you consider the hydraulic circuit for the PTO, the PTO pump is sucking oil out of the reservoir at a little less than atmospheric pressure. As the PTO pump is moving oil, pressure is generated by the resistance of the hoses and valves to the flow of the oil, as well as the energy losses in the moving oil(wasted work), and by the brush cutter motor (useful work). There is also some oil that leaks past the PTO pump, and some oil that leaks past the brush cutter motor, both leading to reduced power transfer (more wasted work). If there is little or no work being done by the brush cutter, the pressure between the PTO pump and the motor will be low. If you measure the pressure between the PTO pump and brush cutter mower when the brush cutter stalls, it should be at 3000psi, or whatever your PTO relief is set at. This is an important point- the pressure in the PTO circuit will be low when there is little work being done. As the workload increases, the pressure will rise. In an ideal world, when the pump is running at full speed, it will be able to generate something like 3000psi at 15gpm to transfer all of that power to the brush cutter. The fact that this heats the oil in the tank pretty rapidly tells you about the energy wastes in the system.
To the extent that your PTO relief doesn't kick in and you don't reach 3000psi at a stalled motor, you may have inefficiencies or wear in the either the pump/motor. Some hydraulic motors have built in bypass circuits for stalls. I don't know what your brush cutter design is. To the extent that your oil is thinner than the motor/pump specification, you may have more leakage, lower maximum pressure, and reduced power transfer.
Does this help?
Finally, if you plan on leaving the pressure gauge in the circuit, you may want to consider putting a snubber on the gauge side of the "tee" to reduce the pressure shocks that are normal in the PTO circuit.
All the best,
Peter