Quoted from vireland:
UPDATE: I checked today and as of now Stern does not have a kit for this ball trap on the Pro, even though the LEs have a piece of plastic that eliminates it installed from the factory. I can make something to match the LE one, but I thought they'd at least offer it for the Pro, too, since they have the same issue with trapped airballs here.
It seems like at least some of the later pros have a big enough gap for the ball to roll through, but ours most definitely did not.
Received a kit from Stern today to eliminate this ball trap on the left side on our Iron Maiden Pro. I think it's the same as the plastic piece on the LEs. Installed it and the ball trap seems to be fixed.
Kit consisted of:
830-1022-43 (Long plastic piece, might have been -48, can't tell)
237-5871-01 (two small phillips screws)
254-5008-12 (taller hex standoff)
254-5008-03 (shorter hex standoff)
237-6172-00 (long phillips screw)
254-5000-12 (round plastic spacer)
There's another ball trap somewhere else, but I haven't SEEN it, I've only HEARD it. I'll report when I find it.