ah, now I understand your problem (it was not so clear from your description to me).
check your OPV settings and definitely check one-way valve.

After purchasing a new check valve, and going to install it, it became apparent that the other end of the manual fill lever was not in good shape. (oddly, this was not rebuilt when the LSM was rebuilt, thinking it probably did not need it... that was an incorrect assumption).

Here is the internals of the old check valve. There is no way this valve was not leaking, or even operating properly. The spring is stuck inside the end cap, the valve on the other side is also stuck, and beating on it would not release it.



At this point, reassembly is waiting on two gaskets. We shall see how it behaves after the manual fill is re-installed.