Why do my rear caliper cable becomes loose after some squeezes in the brake lever? 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.
If you fasten the cable anchor with the quick release at its "tight" position and then it's able to slip into a "loose" position, that sounds like a likely culprit to me. I haven't encountered a misbehaving BR-5800 QR yet so I don't actually know what to tell you there, but I think either the silver part in front comes off and is covering a bolt head that holds the whole thing together, or the whole anchor assembly threads in and out of it. I would investigate that and get the issue you describe taken care of either way. You want those dots to have zero ability to come unaligned unintentionally.

Other possible causes of the cable slackening issue:


The cable is slipping from the anchor bolt.
You've got braided    housing that's bursting through a ferrule,
possibly the hidden one    going into the brake lever. Or it could have been installed without a ferrule. (Coil housings
don't need a ferrule here    but braided/compressionless ones do.)
Someone installed a poorly fitting ferrule at the brake lever end
and it's gradually getting wedged further in.
The cable is in the process of fraying apart    somewhere.
There's a rubber cable doughnut or three caught in a    housing stop
getting gradually squished more and more.
The piece of housing going into the brake lever isn't long enough, and as you squeeze the brake, it's getting worked in further as it gets increasingly kinked where it leaves the tape.