I Agree. bleed brakes. I'm new to the whole 110/1200's scene, in fact I'm new to most DIY mechanics. but the first thing I did when I brought my datto (b110 4 door sedan) was replace the master cylinders in the front brakes. not really knowing what i was doing I was left with exactly the same scenario you describe. problem was that I had reconected some of the brake lines incorrectly (not tight enough fluid was leaking) So I would suggest checking similar with a caution to not overtighten.
Cheers, hope all goes well.