I know that this should be common knowledge for everyone here, but its been so long for me that I forgot :o

I remember that when your brake pedal is very mushy, and goes straight to the floor when trying to stop, that there is air in the lines and they should be bled, but what does it mean when the brakes are very stiff and hard to push while trying to stop?
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Could be a bad brake booster.

lack of vacuum to the booster could be a possibility.

If you just put a new system in, make sure that there is a seal between the MC and the Booster. The piston arm from the Booster to the MC goes through this. Start the car up and listen by the connection between the two for a hissing sound.