Long time Shimano brakes user, I've recently (few months ago) got a new MTB with Sram Guide R brakes. At the beginning those felt great - not as "biting" as Shimano SLX / XT I'm used to, but feeling solid with great modulation.

Unfortunately after some time (few weeks) a rear brake started to get mushy / spongy. This started to happen after long rides with steep descend where lots of braking was needed (nothing excessive, though...). I've assumed that this is classical "I need bleeding" sign and did bleeding - saw air bubbles coming out, brakes started to feel good again. Was hoping that things are good but after another week of hard riding the problem returned. Another bleeding, another good week and the problem was back again.

After bleeding those brakes those brakes 3 or 4 times and getting only about a week of good performance after each bleeding I've assumed that I got defective brakes and replaced them with a new (same model, Sram Guide R). And... guess what - I got 2 weeks of good riding and good feel and the rear brake starts to get mushy again!

At this point I'm super puzzled and really don't understand what is going on. I would appreciate any pointers that would help me to diagnose the problem.

One more observation - tying a brake lever for a few hours (like if it would be pressed when using a brake) "returns" the good feel of the lever for a while...

Totally puzzled, looking for any insights / diagnosis ideas...

  • If you find yourself having to bleed the brakes every two weeks to release air, you either have a leak which is causing air to enter into the system or your brake fluid could be low, as fluid levels drop the void gets replaced by air. – Dan K Jun 29 '19 at 20:41
  • When tying the brake lever for a few hours fixes it, I suspect air in the system is being forced out - presumably to the reservoir. When you operate the brakes, the pressure side fills with fluid, so they feel all good. When you ride, the air bubbles up in the resouvour, and makes it way into the pressure side of the system. Are you absolutely certain your getting all the air out? Also (sorry - have to ask), You are using DOT fuild not mineral? – mattnz Jul 1 '19 at 3:47
  • maybe using old DOT fluid? Or skipping the stage of creating negative pressure in the syringe to draw out the air bubbles? – Andy P Jul 1 '19 at 7:58
  • Definitively using fresh DOT 5.1 fluid from SRAM here so the issue shouldn't be incorrect / old brake fluid - but good call forcing me to check basics! I was suspecting a leak as well and this is why I've replaced the entire brake... The only working theory I've got now is that the old brake had a leak and the new one wasn't bleed properly when initially assembled (I've put it on a bike straight from the package, without bleeding first).... – pkozlowski.opensource Jul 1 '19 at 13:19

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