Check the manifold.
Love that technical term "manifold", nothing but a cheap valve mounted on a piece of angle aluminum in the bilge. At least that is how my H37C used to work. I found that when turning the knob from one tank to another I got less flow from one than the other. Then I started getting a lot less flow from either. Turns out the knob and shaft were turning but the internal "thingy" was not. I took it apart and it becmae obvious. So I replaced it with a simple 'T' and normal pressure returned. Now I can fill both tanks from either side. The only possible issues are the water changing sides when well-healed and having to check levels more often.