Those are notoriously sloppy valves and many of us have replaced them with aftermarket controls sold by Affordable Tractor or RanchHand Supply in order to get smoother functioning and also float and regen, features the Chinese valve doesn't have. That said, I'd suggest you get an assortment of metric O-rings from Harbor Freight or similar and take the offending valve apart, clean it and replace any suspicious-looking O-rings. Definitely take pictures as you disassemble it so you will know how to re-assemble it, and be careful of springs that may go flying out when you unscrew the caps or pull things out. Hydraulic valves are fairly simple in design and function and they probably used standard metric O-rings rather than some oddball special size.
