Just some last minute thoughts before you get a replacement valve.

Are you sure it's plumbed in the right way? I don't know how hard that would be to do, it's been a long time since I had to work in that area. I see an old post on another 240 forum where someone managed to plumb it in the wrong way and it would "leak". The MTC design appears to be a cross shutter slide type valve. The shutter might not seal as tight to one side of the chamber as the other depending on the direction of flow. I've heard of similar things with backwards flowing heater valves in other cars where the valve plate (flap) is on an off-centered axle or seats at an angle. Those designs easily permit reverse flow, in fact sometimes it's an intentional design to allow coolant pressure to balance on both sides of the valve due to fluid expansion/contraction in various parts of the system.

My other thought was a large chunk of scale or whatever getting into the valve and preventing full closure. The MTC shutter design could easily be prone to something just sitting in the bottom of the chamber and keeping the shutter from full travel to close. When you remove the valve, do give it a good flush in both directions to make sure there isn't something caught inside. You haven't ever put Barr's in it have you? That stuff can cause all kinds of such issues.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now