You dont say if you have checked the fluid level so I am assuming it is low. Internal leaks in the pump end up inside the engine and you dont see them.
If you have a Leyland you should join this forum: Peatland Export Parts - Home
I have a Nuffield and have belonged for years. You cant get better info and more help from people around the world.
I will pass along info from my 465 as I suspect it may be relevant to your machine. Note the need to fill the system at full right lock with engine running. Use GM transmission fluid called Dexron.
Many people fit a hose to the reservoir plug and use an external power steering reservoir from a car or truck to allow you to see the level easily and refill promptly.
Dave p