Master Cylinder issue???

Was out and about in my car over the weekend, 70 Duster and had issues with braking yesterday. The car has factory KH Discs up front and Drums in back and no power assist. I still have to check the fronts today when I get home but I adjusted the drums and bleed the rear brakes. Saw no sign of brake fluid and the MC is full. The pedal was solid at times and a minute later it was traveling close to the floor and my dash brake light even came on. I'm thinking my 2 year old Rock Auto MC has bought the farm already. If that is the case I'm hoping to find a better quality one and one that looks correct for a 70 340 Duster. Should both the reservoirs be equal in size or should they front reservoir be the larger of the two? Thanks in advanced