All the stuff people have been saying about brakes in this thread is valid. However, with your heavy clutch, I suspect you may have a different problem that I also had for quite a while. Brake problems can be triggered by a clutch booster that is not working. A non working clutch booster can release vacuum, so when you do a hard brake with both clutch and brake held down, the vacuum leaks out through the clutch booster and ALSO takes away the boost from the brake booster.
Try plugging the hose going into your clutch booster and see if your braking improves.
Try plugging the hose going into your clutch booster and see if your braking improves.