leaks,I just replaced my brake master cylinder and the brakes still feel soft. What do I need to do after I replace the master cylinder

I just replaced my brake master cylinder and my brakes still feel soft. what do I need to do now after replacing the master cylinder. also I have a check engine light on my dash now. I took it to be read and it states brake booster leak. what could that be?

My car has 240000 miles.
My car has an automatic transmission.