You do not need to change the brake fluid as long as you do not open the bleeder screw. Pushing the piston back will simply displace the fluid behind the piston back to the reservoir. Just as long as the brake fluid is be changed at least every 2 years.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record