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DOT 3/4/5.1 is a air moisture sponge. Anytime I have an open brake line for any length of time over 10 minutes I will put a plastic baggie over it and rubber-band it tight as I can. I try to replace fluid every two to three years.
I am starting a GM disc rebuild. I have rusty looking fluid that is at least 8 years old. The master cylinder is covered with rust inside and out. I am pulling it tomorrow and running an electrolysis de-rust process. While that is running I plan on rebuilding the brake booster. I hope to purge the existing fluid out the closest caliper. I will be rebuilding all 4 calipers with new seals and red paint. I hope to have this completed over the next two weeks.
The proportioning valve will cause problems with air pockets if you do a full system drain. I have a similar system to the Harbor Freight bleeder. Mine has the dial to see how much pressure you have on the system. Another option is a power brake bleeder. Check your O'Reily's or Advance Auto to see if they have that on their rental program (deposit and after return the deposit is refunded). The power bleeders do a better job clearing bubbles in the line and proportioning valve.
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Tony
NE ATL
'04 XLT regular cab 3.slo stepside
Semi retirement