I've got two sets of skis right now, both with cast, a well-reputable shop completely mounted one pair and the other was mounted by yours truly... the brake retainer on the pair I did has been super stiff/tight, to the point where its hard to unlock. The set from the shop has been flawless so I think it really comes down to the quality of mount and use of the supplied template from cast. For your case of it being too loose and popping out I'd recommend maybe tightening the two torx screws on the side of the brake retainer to see if that helps or maybe seek out different mounting holes which would be a pain. It would be nice to see cast design the brake retainer to be able to slide on the two vertical mounting screws so that you'd be able to adjust the fore/aft position of the whole unit. I tightened up those torx screws when the retainer loosened up and was flopping underneath my boot while in alpine mode, so maybe that trick could help you in your case. Cheers!