It's best to open it. The fluid in your caliper is contaminated and deteriorated from the constant heat cycles brakes are subjected to. I have done it both ways. If the bleaders look rusted shut I say just push the Pistons open with the cap off of the master cylinder.Im really hesitate to open my bleeder valve because I've heard stories of it just snapping off. Do I have open the bleed valve when compressing the piston, or can I just open the master cylinder?
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