[Juular]

Agree with everything said about having a go at rebuilding the calipers. It's not challenging if you can get the pistons out ok, and using the cars own hydraulics to do so is by far the easiest way.

Pulling the caliper off and gently operating the brakes will also tell you whether the pistons are stuck, and if the hydraulics are working at all.

The rear flexis are very easy to change providing you can dismantle the connections. Sometimes it's easier just to chop everything off and rebuild it with new unions and flares. There's plenty of brake line spare to do this without making things too tight back there.

Edit: it is also very simple to rebuild the existing master cylinder if you really need to. I'm going to say you don't since the front brakes work. The rebuild kits are only a few pounds and the hardest part of the job is bleeding the system, which requires a couple of goes round each corner, or use an ezibleed.