We had the clutch master cylinder (running DOT-5) fail after 22+ years. Disassembly showed the main seal had turned to black goo. I have only seen this happen with rubber parts previously used with DOT-3/4 being re-used after converting to DOT-5. I suspect that the aftermarket clutch MC had been factory lubricated with DOT-3/4 instead of the red rubber grease. Regardless, to be safe I changed ALL the rubber brake seals and hoses on the GT6 since they were the same age as the clutch components. There was zero damage evident on any of the brake rubber parts used with DOT-5.
My point is, if you completely flushed and washed the old system out, replaced ALL the rubber bits (hoses, seals, PDWA innards) that had ever seen DOT-3/4 then you can expect years of good service from the DOT-5 fluid. When I have started with all new rubber bits and clean pipes I have never experienced any of the reliability or driveability problems you see listed on message boards.
: fwiw, my brake system is effectively new
: all around. I dumped Dot 5 in and
: haven't noticed anything awry...
: Granted all I have driving under my
: belt at this point is a few laps around
: the neighborhood @ 25mph for about 10
: -ish miles.