Hi, 1991 Volvo 240 Auto Base Model Non-Abs.

I've been having an intermittent issue with my braking and am hoping someone has had a similar issue or can point me in the right direction....

Once in a while when I depress the brakes the pedal travels to the floor...release brake and depress and full braking again. right away thinking it's the Master Cylinder by not 100% sure as:

1. This happens with no discernible pattern (ie gets better & worse over time) and temperature, amount of braking, length of trip, length of issue persisting - none of it seems to be consistent. Some days / week no issue whatsoever, then one day have 3 - 4 episodes.
2. I may be crazy but if any pattern, seems to happen more when I travel downhill for a period or after a period of extended steady acceleration?
3. If I stand on the brakes when no issue is apparent, it doesn't seem to diminish and the pedal stays firm...
4. Has been going on infrequently over the past 6 months and don't think it has gotten any better worse, just still happens from time to time (Not an easy feeling driving sometimes so yes, need to address)
5. Not losing any fluid

Thinking master cyl. but then think also may be a Vacuum problem or booster issue? Wheel Cylinder?

Appreciate any advice as want to fix but don't want to just start throwing parts at it (but if going to do it, do it right & not just put master cyl on to find after bleeding etc. it was booster all along)