That one is easy on a car of that era,
Your piston is dirty inside the caliper, it's a simple remedy, just push the pistons back as if you were changing the pads, the more worn they are the better for this, then pump them back up without removing the pads, "brake res cap off obviously" do this 5 times and you should be able to feel the brakes clamp on better, whoever told you the cable needed adjusting is having a laugh, the only time the cable needs tightening is if the handbrake isn't holding on a drum assembly,
Make sure you anti seize the pads where they need to slide a bit, same thing with the guide pins as one of those might be sticking, and wire brush the whole lot off where there's surfaces meeting each other not the disk or pad friction surfaces though
Looking at the pics it looks like one side hasn't worked for some time as there's no ridge.