jim. if you dont already have a 210 service manual, not an operators manual, but the real thing, i can send you one. it has a picture of the valve, but not the location.
the valve is under the levers you work to run the system.
you can remove and replace the valve, but its one of those you may have to have to rebuilt to have it worked properly. you can replace the needle valves, but the seats will have to be checked also. hwh has the proper tools to do this.
email me at firstname.lastname@example.org
and i will send you a service manual. it is much more detailed and has all the instructions to do the replacement.
i work for hwh corp. i do not sell parts on this forum