I am not familiar with repairing a spool valve on these machines but have considerable experience dealing with rusty parts you need to save. This might be a good place to try Evaporust{a low Ph. solution}, it will eliminate only the rust and not harm the parent material. Harbor Freight carries it by the gallon for about $25. The part must be clean and oil free, read the instructions first before using, it is amazing stuff. Some tips for using it efficiently are to only use a container big enough to hold the part you are working on and do not put it back with the unused product when done with it. As the rust comes off the part it will appear as small flakes in the solution. For heavy rust you can remove the part occasionally and knock off the loose stuff to help move the process along. As more rust is removed the solution will become weaker, on occasion i will clean it through a coffee filter to get the rust bits out, it does not care if the rust is on the part or in the solution. This extends the life of the product in use. This is not a big deal on a small part, on larger ones it will give you the most bang for your buck. Keep in mind that once you use some, do not put it back with the unused stuff, that will degrade it's performance. I think you will have good luck with it on your rusty valve.