Before you order any parts, please talk to someone who knows that tractor's hydraulics. I don't believe there is any kind of priority valve on that tractor. You won't be able to run the fan motor and lift the planter and markers at the same time.

I just found a bulletin where Deere says you should use a pto driven pump instead. Quoted:

Complaint or Symptom:

6003 series tractors are not compatible with hydraulic motors.

Problem or Situation:

Due these tractors having an open center system, it is not recommended to install hydraulic motors since hydraulic pump would fail.

Solution:

In order to run hydraulic motors on these tractors a PTO driven pump should be used.

For planters the following bundles are available :

• Bundle BH78508 (Auxiliary 540 rpm PTO-drive hydraulic system)
• Bundle BH78873 (Auxiliary 1000 rpm PTO-drive hydraulic system )
• Bundle BH78741 (Oil cooler)

End quote.

There are ways to make hook a planter motor to an open centre system, but it involves hooking between the pump and the tractor's valve stack. Basically, you run the oil from the pump to the planter motor, with a diverter valve to the valve stack (it sends the excess oil the motor doesn't need to the valve stack). You control the fan motor speed with the diverter valve. Depending on where the system relief valve is, you may have to add one to the supply line (before the diverter valve) in case the hoses disconnect. The relief valve would dump to sump if it trips. You also need to hook the hoses together when you disconnect the planter. Oh yes, the fan motor runs all the time the tractor is running.

How do I know all this? I once had a customer who wanted to be able to switch between open and closed center tractors and didn't want to have a pto pump. Neither the tractor or planter were Deere, but the principles are the same. By the time all this was put together, the labour and material came a long way toward the cost of the pto pump. And that didn't include anything for my time researching how to make it work.