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I got some 4 Piston Shimano XT brakes, M8020 calipers with M8020 levers. My rear brake is acting up. On first pull, it feels great. But, if I continue to release and pull quickly, the brake gets spongier and spongier, only giving power when it gets near the bar eventually.

I bleed the brake a few times and changed the pads. But, no avail. The bike mechanic in me says it's gotta be air in the brake. But, I can't find any. Ive replaced the fluid completely and there are no bubbles. I did change the brake line last year after it got pinched then punctured in my frame.

I'm so dumbfounded. Any ideas?
 

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could be a bad hose. remove entire brake assembly and bleed in the full vertical position. brake lever/master cylinder at the top
 

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I got some 4 Piston Shimano XT brakes, M8020 calipers with M8020 levers. My rear brake is acting up. On first pull, it feels great. But, if I continue to release and pull quickly, the brake gets spongier and spongier, only giving power when it gets near the bar eventually.

I bleed the brake a few times and changed the pads. But, no avail. The bike mechanic in me says it's gotta be air in the brake. But, I can't find any. Ive replaced the fluid completely and there are no bubbles. I did change the brake line last year after it got pinched then punctured in my frame.

I'm so dumbfounded. Any ideas?
When you bleed is the bike tilted up so air tends to go to the lever?

did you put a syringe on the caliper and push fluid up as well as pull fluid?

After you take the funnel off the lever, is there a meniscus of oil? If so you can put the sealing bolt in ride around for a bit, take the sealing bolt out, and drip a drop or two of the mineral oil and put the bolt back on.

When you replaced the line originally did you reuse the olive or did you put a new one in? You really should use a new olive. Is the bolt that crushes the olive in really tight?

Some people have had success with pressurizing their brake system. Pressure the system lightly with a syringe on the caliper side then closing the nut to seal the system. I havent found that to be necessary, ultimately you just have to get all the air out.
 

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If you are absolutely sure that there is no air in the system and that there's no leaks in the system the only other thing that would cause what you are describing is failed internal seals in the brake lever.
 
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