When I flush brakes that haven't been flushed in a long while I like to pull the screen out of the master res, suck most of the fluid out with my Pella pump, then wipe as much of the goo in there off as possible, being sure not to leave any chunkies that could get stuck in the ABS system. I then refill and bleed with cheap DOT 3 fluid (new DOT 3 fluid has a higher boiling point then ~3 year old DOT 4 fluid). Then I drive for a while as all the crap still sitting in the system gets absorbed by the DOT 3. Then, I rebleed with some quality DOT 4.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.