This is definately a vacumm issue, sounds almost like the A/C is over boosting the vacumm. If you loose vacumm the pedal usually becomes rock hard.

If you don't know how to trouble shoot the vacumm lines a true service manual from www.helminc.com may help. Else have the local dealer or mechanic service it for you. It could be failed part on the brake booster but not always. Usually the bell goes but that creates no booster vacumm.

It could simply be a check valve but I haven't trouble shot the Accord system recently so I can't give you an exact solution.

Your engine creates lots of extra vacumm in the intake manifold, engineers have used this extra vacumm to run lots of features, power brakes and cruise control too. With the AC on does the cruise work?