Verify that water is leaving via the filter waste. You can do this by drilling a 1/4" hole in the underside of the waste pipe -- and you can patch it later with a piece of rubber from a Fernco coupling and hose clamp, or by cutting and coupling there.

If it *IS* leaking significantly there, then is it IS a multiport valve leak. The question would be: why is a new multiport valve leaking?