Check how many inches vacuum you have going to the power booster.

Step on the brake and check (again) how many inches vacuum you have going to the power booster.

Should be not less than thirteen inches of vacuum.

You may also have bad brake flex hoses.

Brake:
http://www.peachparts.com/shopforum/showthread.php?t=142411



Rebuilding the Vacuum Pump on the 617 Engine
http://diymbrepair.com/easley/Rebuild_vac_pump.htm

1982-240D-Vacuum System
http://www.peachparts.com/shopforum/showthread.php?t=105201

Vacuum pump lesson learned
http://www.peachparts.com/shopforum/showthread.php?p=872760#post872760

Vacuum Pump Gasket Sealer & Torque
http://www.peachparts.com/shopforum/showthread.php?t=138153