When I step on the brake pedal I hear a hissing sound and the pedal goes all the way to the floor and the brakes barely stop the car. This doesn't happen every time but most of the time. Any ideas of what this could be? My husband just put new brakes on the car right before this started happening.
If pumping the pedal until it's hard doesn't permanently fix it:
Because you say the pedal went to the floor and you heard a hissing, my guess is the master cylinder is bad or you have a brake fluid leak. The hissing is likely because the brake booster is being pushed farther than it was ever expected to (pedal to the floor) so the rubber membrane inside is slipping temporarily and causing a vacuum leak.
Either way, this is nothing to mess around with. Don't drive on it; get it addressed as soon as possible.
Try just pumping the brakes several times with the engine running and see if that doesn't help. It could just be the calipers need to be filled back up with fluid which will adjust them to where they need to be. When I say pump them several times, continue pumping/releasing until the brake pedal gets hard.