310/340 cat pump valve issue

[Ric]

One of the valves on my 340 cat pump keeps losing the o-ring off it. Is the valve seat washed out?

 

[I.B. Washincars]

One of the valves on my 340 cat pump keeps losing the o-ring off it. Is the valve seat washed out?

Most likely the area of the head/manifold that is adjacent to the o-ring is washed out. Look in there and see if it is. There's nothing mysterious about it, you can see and feel it.