Problem with an under the floor master is that it can end up being the low spot in the system.
Mounted up on the fire wall, there is positive pressure in the lines due to head pressure in the lines. If the master is mounted it the level of the wheel cylinders or have lines running higher than the cylinder, fluid runs back and there is no residual pressure in the wheel cylinders.
Install a residual pressure valve in each line, close to the master.
Secondly, With lines higher than the master, air traps are common. Pump and bleed won't flow enough to clear them out. You have to use a pressure bleeder. Rent or make your own, not difficult.
