you're going to spend more on testing than the cost of new brake fluid. it only takes a couple pints to flush the system. unless you are using Motul racing fluid, which is very unnecessary unless you are racing the X every weekend, at which point you need to change racing fluid more often anyways.

every 2 years will prevent you from having corrosion problems in the brake system. That's all you should be worried about. It takes a good 3-4 stops from triple digits before you have to worry about a lower boiling point caused by moisture content.

Buy a pressure bleeder (Motiv) for $50~ and a few pints of brake fluid. Your return on investment is 1 brake fluid flush. Every flush after is saving you $70-100. Recycle your old fluid at the parts store or the hazardous waste collection center. Don't mix with other fluids as it makes those other fluids no longer recyclable.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i