Check your rear brake hose for wear as it goes through the last guide on the chainstay before the caliper. I’ve had two standard hoses fail there due to wear which luckily I found in the garage on a maintenance strip down rather than in the Alps a couple of weeks ago. Early symptoms are the lever bite point starting to move about and then the lever slowly pulling to the bars. I could of made up a small sleeve to protect the hose but went braided as I had to replace the hose anyway.