Trouble with XR400

Hi

A couple of suspension questions please.

Besides the GS, I also have a 1998 Honda XR400 that seems to have lost all damping on the rear shock absorber. Does anyone know if these units are serviceable?

If so does anyone have any recommendations or if not any suggestions for a replacement?

Can you tell me what the valve on the bottom of the reservoir is for?

Many thanks

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