Brake issues

SO THIS MORNING i went out to the aspen and drove to work omw i could feel the brakes were acting diffrent when i got to work there was fluid missing from the back resavoir closest to the booster i drove home and it used more i havnt driven it at all since.

the problem is the car stops then the pedel hits the floor and it starts rolling like the brakes were never pushed. it just slowly u can feel ur foot going to the floor i need my car for monday but aint driving it in this condition..... i ruled out the back wheel cylinders in the rain i pulled the back wheels off and looked just dusty not wet at all i didnt pull the front off i got to cold.
i jus cant figure out where the fluid is going or coming from i orderd the only master cylinder in town a remanufactured pos from oriely im going to order a new one as well but they cant gettem till a week

am i n the right direction someone told me the rear main was leaking on my car cuz between the booster and master there is a dust trail from stuff sticking to the fluid and that it would start filling the booster with fluid and then i would need both replaced?????????

i need help its raining for the first big rain of the year and i got no brakes