Silicone (DOT 5) fluid need not be put in a pristine system as there are no chemical compatability "issues" with mixing a little bit of glycol based fluid in the silicone.

The trouble is that water (and glycol based brake fluid) is entirely imiscible with the silicone fluid. That means any water or water contaminated or even "dry" brake fluid in the system will tend to settle into the low points of the tubing and collect there. This can lead to local corrosion and brake pipe failure or nasty spongy brakes if the colleciton point is at or near the calipers or wheel cylinders (from heating and subsequent boiling).

OTOH a decent flush on installation and a couple subsequent bleedings over the next few months tend to get rid of all the troublesome old stuff without problem. Then it can be run "forever." You still need to flush out accumulated water from condensation in the MC from time to time but it is essentially a no-service alternative.

With "regular" brake fluid I like to "spit" the bleeders annually to both ensure there are no nasties at the caliper/cylinder and to ensure the bleeders aren't frozen.

Were I you, I'd assume the wheel cylinder is toast and get two new ones. One each left and right!


--
Mike!