Hey all you gurus out there, can you help me?

I bought a '91 accord wagon ex, excellent shape, well taken care of. Only problem is that when the A/C has been on for awhile and am in traffic on freeway (brake, move, brake, etc), the brake pedal goes clear to floor and brake barely catch. Just had brakes replaced, checked all brake lines, no fluid loss, wheel cylinders ok. Master cylinder never loses fluid. When A/c is off, never have problem.

Any thoughts on cause of this loss of braking? Wierd.

Thanks for your help.