About Mehrwan's Jinma JM-304

I got this Jinma 304 Tractor with loader and backhoe. The relief valve had busted O rings which I replaced with new ones but it seems that got busted again because I can’t move the backhoe at all! A few questions: 1- Are the O rings generic or are specific to this part. 2- if not generic, where can I get a rebuilt kit for the release valve? 3- How is it possible to adjust the pressure in the valve? ( There is no set screw on the valve). Thanks for any help.