Are you really saying that bypass fluid from a worn piston seal can not leak into the crankcase?
I sure thought that must all 3pt internal cyl's backsides were open to the sump.
If a piston seal is perfect, the only way fluid can get back to the sump is through the descent valve, or a bad valve spool, and yes, it will block the fluid if you want for transport.
It is also used as a variable descent device.
If you close off the descent valve, you close off the 3pt valve as being the problem.