So last year I noticed my brake pedal was going to the floor when the A/C was on at stop lights. I installed Accord wagon front calipers, installed rear disc, and a 40/40 proportioning valve. Now this summer its doing it again. The pedal likes to sink down when I sit at a stop light.
What in the hell would make that happen? Its not a brake component, I don't see any leaks anywhere.

 

I can't think of any reason why the A/C would affect the brakes at all. Maybe just a coincidence. I guess you could possibly have moisture in your brake fluid that is evaporating when it gets hot and creating water vapor which will give a soft pedal. That's all I can think of. Your pedal shouldn't go to the floor unless you have a leak or the master cylinder is bad. I would flush/bleed the heck out of the brakes. Make sure your master cylinder reservoir cap is sealing properly and not allowing the brake fluid to absorb moisture (which it loves to do). If in doubt, replace the cap. For a complete flush and fill, buy new brake fluid (in pints) so that you know the container has been sealed and is free of moisture (don't just use some that you've had for a while that has been opened previously). This will ensure there is no moisture in the fluid, and you can keep the left over fluid for top offs later. If you buy quart bottles, you will expose all the fluid to the atmosphere each time you open it. Pint bottles allow you to expose a smaller amount to the air so that the other bottle stays sealed until you need it.

 

The brake lines have been flushed and the fluid is nice and clear. I,m thinking its a vacuum line problem. Or maybe the A/C is effecting the idle. I,m confused.

 

40/40 proportioning valve?? I never heard of that. where does the extra 20% go?

 

the idle speed would affect the vacuum applied to the booster, but I wouldn't think it would cause the pedal to fall the floor. have you swapped the master cylinder? you could try revving the engine up a little next time it happens and see if it's an idle issue or not. does turning the ac off fix it after it happens?

 

need a new master cylinder.