If it's done enough times, though, one could almost entirely replace the fluid, right? What's the downside of this, assuming it's done repeatedly? Then, as a way of maintaining, just drain and refill the reservoir at whatever interval.

Just trying to understand....

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You'd never have entirely "fresh" fluid and would remove newer fluid along with older fluid. You'd end up using more fluid than necessary in exchange for a bit of labor savings, but by the time you factor in the number of times you'd have to do it, the labor would probably be a wash.

For example, let's say system "X" contains ten quarts of fluid (ten is convenient here for the math), and let's say that between each exchange of one quart, the "old" fluid stays "old" and the "new" stays "new."

Change 1)

- remove one quart of mixed fluid, replace with one quart of new
- the removed fluid is 100% old
- you end up with 9 quarts of old, 1 quart of new

Change 2)

- remove one quart of mixed fluid, replace with one quart of new
- the removed fluid is 90% old, 10% new
- you end up with 8.1 quarts of old, 1.9 quarts of new

Change 3)

- remove one quart of mixed fluid, replace with one quart of new
- the removed fluid is 81% old, 19% new
- you end up with 7.3 quarts of old, 2.7 quarts of new

... and so on, ending up with these amounts after each change:

Change 4) 6.6 old, 3.4 new

Change 5) 5.9 old, 4.1 new

Change 6) 5.3 old, 4.7 new

Change 7) 4.8 old, 5.2 new

Change 8) 4.3 old, 5.7 new

Change 9) 3.9 old, 6.1 new

Change 10) 3.5 old, 6.5 new

So, after ten changes using a total of 10.0 quarts, you only end up with only 6.5 quarts of new fluid, whereas if you'd changed it out in one session, you'd have 10.0 quarts of new fluid. It would take a long time and many changes to even approach a total changeout. Yes, the proportion of changeout on the real system we're talking about would be higher and you'd approach the asymptote more quickly, but the point is still the same.

Put another way, one job done halfway, plus another job done halfway doesn't equal one job done completely.

not to mention that all the left in new fluid, becomes dirty and contaminated when it contacts and mixes with the old fluid...
John

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