Dashboard lights: ABS, Brake, and (!)

For a couple of months now the ABS, Brake, and (!) lights have been on. There are a couple of things that are affected by this. The brake pedal needs to be pushed harder for them to work, and the battery doesn't gather power when I press the brakes.

Another strange thing is that the lights go off sometimes. They are usually on, but they will turn off for 20 minutes or so every couple of days. When they turn off, the normal brakes and the battery gathering capability is restored.

I've looked at other Prius Chat threads but I haven't been able to find anything conclusive. I've heard that it might be that the brake pads or the 12V battery need to be replaced.

I don't know if it's related, but I also hear a hissing sound coming from the lower front left of the engine compartment. Could it be a broken or disconnected vacuum line?

Thanks in advance for any replies, and any advice would be appreciated.