I have bought this bike one week ago from a friend of mine, and when I checked it, it was clear that the rear brake caliper cable was loose. I could see, under the frame, the steel wire bulging from the lack of tension.
So I thought it was a easy fix, just tension the cable again, adjust the rear caliper and everything works, right? Apparently, no.
After retensioning the cable and adjusting the caliper, with a few squeezes in the corresponding brake lever, the tension disappears and the cable becomes loose again. I don't think the problem is the caliper cable nut, as the cable does not seem to move there.
One symptom it has, is that the lever is much more hard to squeeze compared to the front caliper one. I think that is a problem with the cable/housing, because I have removed the rear caliper from the frame and it does not seem to be blocked or stuck in any way.
Do you think the solution is to replace the cable?
Besides these problems with the cable, I am a little bit concerned that the quick release mechanism of the rear caliper has some kind of issue. It feels very hard to move compared to the front one, and I am actually able to drive it further the locked position (past the two white dots aligned). I don't know if this may also explain the loss of cable tension.
FYI: caliper/levers are 2016 Shimano 105.