I have had similar problems. A very slight chamfer on the nipple helps a bit. It also seems critical to seat the cap fully - I give them a squodge with some long nose pliers - but not too much.
Chap called Tac Foley (Tac) on here sourced a set of SS replacement nipples (Bushnell I believe but they are identical to the originals) for me and he gets then in batches whenever he goes over stateside.
777? never use it - BP all the way for me.
True freedom includes the freedom to make mistakes or do foolish things and bear the consequences.