Uneven brake pad wear

[Ceiran]

Hi, I had this bike second hand and I'm trying to get it back to working order but I'm also a novice, the rear brakes are awful and I noticed the pad isn't contacting the rotor at the bottom, it was more uneven than this initially but I sanded the pad down a bit and rubbed it down with isopropyl, I figured actually there's probably an underlying issue as to why the pad wore uneven in the first place which I should look into trying to rectify before replacing the pads. If anyone can give me a few pointers in terms of what to look at/correct before replacing the pads I'd be really grateful. Thanks in advance for any help.

I believe the brakes are Zoom mechanical brakes with a single piston caliper. (Please correct me if I'm wrong

 

[Pedropete]

Yep, single piston with static inboard pad adjusted by the hex bolt on the inboard caliper face, visible in the last pic you attached. You need 1-1.5mm gap between rotor and static pad so align the calliper, set that gap and then adjust cable tension to set the outboard (moving) pad. Then realign the caliper if needed. Too big a gap between inboard pad and rotor and you'll be deflecting the rotor too far on braking, leading to uneven pad-wear and potential warping of the rotor under aggressive braking.

EDIT: when setting up from scratch, align the caliper so the rotor is centred in the caliper. Then adjust the inboard pad with the hex bolt. If the inboard pad is too far inboard you'll quickly wear through it and end up braking on the caliper body itself rather than the pad.