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Frequently Asked Questions - Circuitry and Function

Q: What size does my hydraulic reservoir need to be?

A: As a general rule of thumb your hydraulic reservoir in gallons should equal the pump output in gallons per minute.

Q: How do I figure out what size my Prince PTO pump is?

A: You can find out what PTO pump you have by measuring the center section of the PTO and knowing if it is a 6 tooth (540 rpm) or a 21 tooth (1000 rpm) drive shaft. Based on the center section width and the drive shaft, you can determine what Prince model # it is you have. Click here to view Prince PTO dimensional data and specifications.

Q: What is a closed center or open center valve?

A: Open center refers to valves that allow oil to circulate at low pressure through the valve and back to the tank or reservoir when the valve is in the neutral position. Open center systems are normally used with gear pumps…aka…if the engine is running, it is pumping oil. Closed center refers to valves that do not allow oil to flow through the valve when the spools are in the neutral position. The flow path is dead headed at the valve. Closed center systems are normally used with piston pumps or pressure compensated pumps.

Q: I just bought a new Prince directional control valve and it won’t make my cylinder extend or retract. What is wrong with the valve?

A: Keep in mind that directional control valves work much like a switch, which simply redirects the hydraulic oil flow path. If the oil is getting to the inlet, not going over relief and has a path to tank, there’s not much that can go wrong with the valve.

Here are some common items to check if your valve doesn’t have any pressure or nothing happens when you move the handle:

  • Always make sure the valve is hooked up properly.
  • Are there any other valves or tees in the system?
  • Always follow all potential flow paths.
  • Is there another path the oil can take before it gets to our valve?
  • Is there a tee in the line before our valve? Is there air in the system or is the oil frothy?
  • A low oil level in the reservoir or an undersized oil reservoir can cause this. Remember the general rule, 1 gallon of reservoir to 1 gpm of pump flow.
  • Is the oil bypassing, going around the cylinder piston seal or bypassing in the motor?
  • Is the pump producing any flow at the required pressure? The flow in worn pumps often decreases or is zero as the pressure increases.
  • Does the valve outlet have a line going back to tank? If you are using power beyond you must have a line from the outlet of the valve connected to the tank and a line from the power beyond to the downstream valve.
  • Check the quick disconnects in the system. If there are any, make sure they are properly connected.


Q: I just put a Prince control valve on and it’s leaking out the back of the valve. What went wrong?

A: There is a very good chance that the outlet line (return to tank line) has been pressurized. Just one tank line spike over 300 psi can cause the valve to leak at either the handle or the spool end cap.