Background - I just took my car out of storage this spring and have a brake issue. Occasionally, the pedal goes to floor while slowing down and I have minimal braking - happens more often when just stopped in traffic. Fluid level is fine in MC. No fluid around wheels. Power brakes, discs on front, drums on back.

Thinking its the MC, but when I looked at it - doesn't match what appears to be the correct MC for disc/drum on a '71 cutlass supreme. Both wells are the same size on the existing MC. "Correct" MC appears to have a larger well in front and different bale across the top.

Did a previous owner change the front from drums to disc and not change the MC? (and probably not the proportioning valve either?) Pic of existing MC is attached.