I'm a little puzzled. The problem you describe is typical of a pump located above the waterline as with an inground pool. Is you pump above the waterline? Let's assume it is and further assume both the pump and filter are above the waterline. My first guess would be the pump basket lid and since that is on the suction side you wouldn't see any dribble and the leak may be so small you see no bubbles when the pump is running. My second guess would be the multiport valve. Gaskets sometimes do funny things. Seal under pressure and leak under vacuum, i.e. when the pump is off. Could be sucking a little air via the waste port. Any way to put a cap on that pipe and seal it to verify? Now, if your equipment is on the ground below the water level of the pool.....I don't understand something and you can ignore what I said above.
Al