Yes it does, when mains pressure drops to zero two things are possible
without the air brake:

1. Mains pressure actually turns negative (very common, just takes one
opening downstream to do this...)

OK, the mechanism for contamination exists.

2. Even if pressure is truely zero, contamination will backflow into
the pipe, and at a surprising rate. Technically it is possible even
with full pressure (this is caused by boundary layer effect where the
water at the side of the pipe or conduit is not moving.) A pipe run
underwater, with a pin hole, will draw contamination into the water
supply. If that 'underwater' water source is heavily contaminated, the
amount of contamination can be significant.

BUT THERE'S **NO** "DIRTY" WATER IN CONTACT WITH THE FILL HOSE!! THE WATER IS
10" BELOW THE HOSE!! HOW IS THE WATER SUPPOSED TO EVEN MAKE THE TRIP UP TO
THE HOSE AND THEN TO THE MIX VALVE, ANTIGRAVITY?!
--
DaveC

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