Good poimts, but I did'nt mean to say that there are springs in the disc system, rather that there are springs in the drum system. If you had no residual pressure with drums the springs would retract the pads until they hit something ( probably bottoming in the wheel cyl.'s), causing you to have to pump the pedal multiple times before the pads contact the drums. I'm not well versed in the auto systems, so I find it odd that disc systems would require any residual pressure. However I happily defer to your experience as I have yet to mess with these disc conversions. I guess maybe if the master resides below the wheel cyl.s, it could drain back with gravity or something. Some calipers I work with have square sealing rings that cock themselves in the grooves, and provide some springing action to return the pistons, but most just rely on the slight highs and lows to knock the pad back in a little. This is why drums used to be preferred for drag racing, because they positively retract via the springs, thereby ensuring no drag.
I re-read the original question, and see that there is a free-play issue. Obviously there must be freeplay at the pushrod, or pressure will continue to rise as the pedal is pumped. Also I could see where a misaligned caliper bracket could cause drag.

Who is John Galt?