It's not the length of the pipe or the amount of extra fluid, it's the extra calliper. A master cylinder will only move so much fluid. The total amount of fluid in the system is irrelevant. It is the need to move the extra piston area of the extra calliper towards the disc. There's a ratio of master cylinder to slave cylinder sizes that needs to be maintained within a tolerance. If the master is too big, the brakes feel on/off as little movement is required to operate the slaves. If it's too small it means the lever/pedal has to travel further to move enough fluid. If it still moves enough fluid to do the job it can lead to a soft feel to the lever/pedal. At it's worst case it doesn't move enough fluid to activate the callipers properly before the master cylinder bottoms out. Which effect you'd get in this case (soft pedal or no brakes) I don't know as the master and slave sizes are unknown. It's perhaps extra critical in a home made set-up as you'll be mounting callipers on custom made brackets. Unless you get the mounting 100% perfectly aligned to the rotor the act of the rotors spinning will push the pads away. This then needs a bigger movement of the master to get the pads back to the rotor and clamping.
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