Lose power when stepping on break??

It would seem that the battery is not being charged. Its possible the new alternator is faulty. First step is to verify proper charging of the battery. You can do this by using a basic volt meter on the battery. It should read somewhere between 13.5 and 14.3. If you do not have access to a volt meter just drive over to a local auto parts store. They can put a load on the system and check the battery and alternator at the same time and its normally free. Alex