There are a couple of things that can cause a rock hard pedal depending on whats been done to the system. If the brakes seem to work well my guess would be the master cylinder bore being too big for the system. Power brake setups use a 1" or larger bore in most cases and manual brakes generally use a 7/8" bore or less depending on the system. the larger bore means more volume of fluid but less overall pressure (the volume being the reason the pedal travels less) and a smaller bore means more overall pressure but less volume meaning the pedal travels further to move the amount of fluid it takes to apply the brakes. (this is just one cause) you also can't rule out a tight pivot for the pedal arm, blockage in the system, a faulty residual pressure valve in the master cylinder or combination valve etc. Hope this helps.
