273 is correct the volume in a drum master cylinder will work fine for discs
2 things...If your brakes and lines are new or in good repair you either have a master cylinder piston that is too big or you haven't bedded your pads.Hard stop 3 times from 60 mph to 30 mph and then when cool haul it down from 30 mph to a stop a few times and you should be good.
Manual discs need a master cylinder with a piston 15/16" or less.
He said he cant get them to lock up on a gravel driveway at about 30 mph standing on the pedal, are you trying to kill the man ?!? Obviously there is something wrong. Change to the correct split circuit disc/drum master, and try it again.
