During a recent "exercise" cruise in my '63 Impala I noticed an extra amount of travel in the brake pedal during normal braking. There was no "spongy" feel to it and the pedal never hit the floor, just the extra travel. My Impala's brakes are regular hydraulic but not power-assist, which means the master cylinder has only one reservoir for all the fluid and that my emergency/parking brake had better be in working order in an actual emergency.
Upon returning home I backed the car into the garage without incident and the brakes did not fail. I made a mental note to check the fluid level before taking her out again. So, a couple of weeks later I was preparing to take her out again on a bright sunny day, remembering to check the brake fluid level first. When I popped off the master cylinder cap and looked inside it was bone dry! That's when I started wondering; is it possible to have a small enough leak somewhere in the brake system, perhaps at one of the wheel cylinders, that will let out just enough fluid but not suck back in too much air during low speed braking? Thereby avoiding complete brake failure because no air bubbles make it all the way back to the master cylinder?
Your brake system's condition is not due to any systemic design flaw that requires part/system changes. It's due to lack of proper maintenance/attention. The system gives 'notice' of problem(s), typically thru the feel of the pedal, grabbing/pulling to one side, and/or 'puddles' of fluid . Waiting for symptoms to occur to inspect the brakes is not something I would do.
Even with no sign of problem, I check the fluid when I check the oil. I pull a drum (a different one each time) about every six months as part of a periodic brake system check. I did that back in the '60s and have continued the process with my current '63.
Such a routine is recommended regardless of the master cylinder. Surprisingly losing approx 60% (if front brakes fail in a dual system) of braking capacity and relying on the remaining 40% is quit risky.
Is having 40% better than what the mechanical brake supplies? Likely yes (I never did any sort of test). But performing proper periodic preventive maintenance has proven (by all the safe miles driven prior to 1967) to provide safe driving.
I'm NOT anti dual master cylinder; just PRO preventive maintenance.
To the specific question:
"... is it possible to have a small enough leak somewhere in the brake system, perhaps at one of the wheel cylinders, that will let out just enough fluid but not suck back in too much air during low speed braking? Thereby avoiding complete brake failure because no air bubbles make it all the way back to the master cylinder?"
Yes. The seals and springs within the wheel cylinders can 'leak' as they begin to fail. Also, within the master cylinder there is a check valve that keeps some pressure on the wheel cylinders to help maintain the 'seal'. Dirt and other contaminants can cause 'leaks' and changing fluid periodically is essential.