I was braking in a downhill, the gradient was steep so I was using both brakes mostly the rear. When suddenly the rear brake shifter went totally soft. I did a full-stop using my front brake only and took a look at the rear, this is what I saw:
The bolt holding the brake cable clamp on the caliper is missing. I had never touched at this bolt. Looking at the front brake, it's TorX head for which I do not even own a key. I looked everywhere around the place the brake gave up but could not find the bolt. It could have been lost way before: I put the clamp back in place and managed to ride using the brake too without it loosening again.
The brake is a Tektro R539. How easy is it to find a replacement. Should I just measure the front one and look up on eBay? Is it worth trying the local bike shops?
Edit, pictures of the front nut:
Edit 2: Went to the hardware shop. By putting the nut against a benchmark it looked like an M3. They were selling none of these though.