My girlfriend has a 91 accord se and the breaks go all the way to the floor when the ac is on and the more you have the ac turned up the worse it is the only thing i can think to relate the two is the is the electrical system. the reason i suspect the electrical system is the radio is lose and when Jarred by shard turns some speakers cut in and out (possibly a short)

 

I would first bleed the brakes but that is probably a waste of time...but it sounds like the master cylinder is gone and needs to be replaced. Its a 2-3 hour fix for under 75 bucks.

 

I would only guess that when the AC is turned on that the RPMs of the engine drop creating less vacuum thus loosing brake booster vacuum and, well no that is wrong, turning on the AC should increase vacuum thus increasing vacuum to the brake booster helping pull the pedal towards the floor, after all, that is what power brakes were desiged to do. Well no that is not correct either, with poor vacuum, regular vacuum or great vacuum the pedal should never go to the floor, looks like a master cylinder rebuild or just buy a rebuilt one ready to go, either way dont forget to bench bleed it before reinstalling it to the car.

Duane in Japan

 

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Duane in Japan</TD></TR></TABLE>

why are u in japan, duane?