[dern]

OK so last night I changed the brakes and rotors on my 96 accord and today while sitting at red lights for to long I notice that the brake pedal would go all the way to the floor I would put the car in N and pump the brakes and its fine any reasons for this thanks

[smoothy]

DO NOT drive the car...have it towed to a mechanic.... your master cylinder is failing and it can suddenly go out at any moment completely leaving you without any brakes. It must be replaced and the brake system properly bled.

[odinn7]

Did you replace any of the hydraulics (calipers or wheel cylinders) when you did the work? Did you open any of the bleeders?

[talaniman]

You could have allowed air into the system when changing the brakes. Bleed them with a buddy's help and go from there.

[TxGreaseMonkey]

Master cylinders can be damaged if the brake fluid is forced back into them. This often happens when the pads are being spread and the piston is forced back into the caliper. It's safer to clamp off the brake line and open the bleeder valve than to remove the master cylinder reservoir's cap. Therefore, I wouldn't be surprised if you need to replace the master cylinder (bench bleed prior to installation)--be sure to use a flare wrench on connections. This is the ideal time to change the brake fluid and bleed the complete system (RR, LF, LR, and RF).