You may run into a closed center vs. open center situation. As I understand it most John Deere tractors are closed center. Don't know about New Holland, just a matter of changed some internal plugs and possibly a relief valve. Found this on another website:
Q: What is the difference between an open and closed system?
A: The terms Open system and Closed system refer to two methods of reducing the pressure on the pump, which minimizes wear and tear.
Open systems are common on logsplitters and most tractors prior to 1960. When an open system is in neutral, an open center valve connects all lines directly back to the reservoir, bypassing the pump. The pump is always pumping, allowing a constant flow of oil without building pressure.
Closed systems are common on construction machinery and modern farm equipment, including most John Deere models. When a closed system is in neutral, the closed center valve blocks the flow of oil from the pump. The oil travels instead to an accumulator, which stores the oil under pressure.