JDeere970, those are nice tractors. 900 hrs is nothing. But time to clean that screen.Here are the two pages from the JD970 operators manual that talk about changing the trans/hydraulic fluid. The location to remove the tubular cleanable screen is shown.
First you have to drain the trans/hydraulic fluid. It needs to be super clean. That fluid is expensive. Roughly $150 for 6 gallons at Amazon or the tractor shop. Use JD's Lo Viscosity Hy Gard for smoothest performance. Also helps the tranny. Cheapo "tractor fluid" is usually too thick for wintertime in that tractor. Sometimes the Hy Gard container just says "Winter" instead of Lo Viscosity.
If your fluid is recent, you can reuse if it is clean and still transparent.
If milky, disgard it and flush with cheap oil. Milky-looking trans/hydraulic fluid means it is holding water in suspension. Best way I know to clean fluid if it is NOT milky is let it set for a few days in a clean container and then carefully pour it off or siphon it out to leave the crud behind. Expect to lose some. Sounds like you won't have to do the main filter again unless it is some aftermarket type and you want to play it conservative.
The cleanable screen is under the cover in the picture. It is a tubular mesh screen. The screws are metric allens I think. You can clean the screen with a tooth brush. Some screens were metal and others nylon mesh. Clean the crud off the outside and slide it put it back together.
There is no gasket there, just an O ring. Lots of places have the CH17154 O ring. Amazon, Walmart, JD..
I think Walmart and Amazon also have the Hy Gard Lo Viscosity Fluid.
Good luck,
rScotty