If the master cylinder is mounted very low (and it probably is) then the builder likely put a residual valve in the lines. Normally one for each circuit. Something to be aware of, there are two common residual valves. These come with different residual pressures depending on application. I've seen 2 pound up to 10 pound. These maintain a "residual" pressure in the line when your foot is off the brake so that the fluid doesn't all try to fly backward to the low spot (the master cylinder). Kind of simplified, but that's how they work.
With residual valves in place, the 2 pound ones can barely if at all be felt when you are rotating the tire. The 10 pound one makes a bit of drag.
Perhaps this is the issue? As long as nothing is heating up, e.g. your hubs or your drums, when you drive, then you likely have nothing to worry about.
angela