Can't tolerate FF mask but jaw relaxes when I sleep

Personally I can't tape cause my skins allergic to adhesives, and the premium chinstrap was a joke, MOST chin straps have to much elastic for me. And won't keep me from relaxing.
Right now i'm not sure of the one i am using. it has a shaped chin area that cups the chin and a NON-elastic strap. But its a single strap which even worn under my swift LT slips and slides so I added a extra T type of strap that goes across the back of the head. And that has done the trick.

As to the training of your jaw muscles, yes its possible. I went for 12 out of 15 years of using CPAP without a chin strap, till the last few where I start to fall asleep and relax a bit then my jaw suddenly snaps shut. usually with me biting my tongue or cheek in the process. Which is why i started using a chin strap after all these years.

zipo, I understand you don't want to use tape. You might want to check out some of the links about a "DIY" thingy made from a couple of boil'n'bite silicone mouthguards.

If you can make one of those to keep your tongue shoved either up or back, in a sealing position, it won't matter if your jaw drops or mouth is open. The position of the back of the tongue is the key to keeping air from gushing in, or out, the mouth.

While sitting there right now, start breathing through your mouth. Then swallow slowly.

As you start to swallow, you can feel the way the back of your tongue closes up against the back roof of your mouth. A "Do It Yourself" device that keeps the tongue pushed back in that position will keep CPAP air from detouring from the back of your throat into (and out) your mouth. It will also keep you from breathing in through your mouth.

Read about making a DIY mouthguard here:
LINKS to: Mouth leaks - Air Leaks - Tape - DIY Guard
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Welcome to the forum zipobibrock5e8 !
I chased leaks for four months, tried different headgear (that made a difference). What works great for me is a sports mouth guard (the boil and bite kind) and a good old Ace bandage. Leak rate went from an average of 22 L/min down to 4 L, or less on most nights with many 0.0 nights.
Make a loop from the crown of your head down to the chin and back.This will lay across your cheeks in front of your ears, or partially cover them. The Ace I have is the self sticking kind and leaves a residue on my chin when I sweat...in case you have sensitive skin. The headgear goes on over the top of the Ace and together they seem to help each other stay in place.

Good luck and hope you have a great solution soon.
Cindy