[1] If you have a hard pedal, then you do not have air in the system.
[2] System. THE most important word. 'Cause the brakes work as a system.
[3] Disk brakes require about 50% more line pressure than drum brakes. So if you are using a drum brake M/C with disc brakes, the bore is probably too large & will result in a hard pedal & poor braking performance. The solution here would be a smaller M/C. These are general principles; you would need to check with the disk supplier which M/C you need & the M/C for power brakes will likely have a bigger bore.
[4] This is a case of measure twice, cut once...
