If the brake pedal feel is firm, but the car doesn't want to stop, this could be a sign there is something wrong with the brake booster.  Check the brake booster vacuum line, because if the brake booster does not get adequate vacuum (under braking), the brake booster will become ineffective.  Ensure the check valve is installed correctly (arrow pointing toward engine).  If the check valve is installed backwards, the brake booster will get no vacuum and brake performance will suffer.  If the check valve is not installed, boost from the turbo can damage the brake booster itself.  Damage to the brake booster will degrade brake performance.

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