if your brake master cylinder has never been changed, then i suspect the seal is starting to go.

and the reason why it coincides with you using A/C might simply be because the engine runs hotter when the A/C is on. when my BMC started going, during hot days (90+ degrees), the pedal would sink. but 80s or lower, i could brake normally. but eventually, you'll completely lose the seal and you won't have any pedal pressure.

now you say when you turn the a/c off, the brakes work fine. you mean to tell me once you start losing pedal pressure and you turn off a/c, you regain pressure immediately? if that is the case (though i find it hard to believe), something else is screwy.