Luath, maybe to help you understand your tractor.
The "spools" are components inside the hydraulic valve. The valve (loader valve) is the part with the handles attached to loader brackets (usually the "towers").
The spools are moved forward and backward by the levers on your valve as you move them. One one end of the spools is a pin with the levers, on the other end are some seals, springs, small round balls to make the "detent" (for float) work. You can't see this end. This is the end that is effected by water that gets trapped in the housing.
You can google "two spool hydraulic valve' and get a picture of what it looks like. There is some variation in valves, but pretty much all operate the same.