Question about rear brake (pads and pistons)

[BRB96Z34]

My rear brake started to squeal a couple of days ago so I take off the inspection cover to have a look.  The inner pad has plenty to go before reaching the limit line but the outer pad is almost ready to be replaced.  I tested the brake with the cover still off and only the outer pad is compressing.  It's almost as if the inner pad is siezed.  What should my next course of action be?

[scratch]

Pull out the retaining pin and both pads. First, I would see if I can push in the piston that is not working, if it moves, good, it may just need to be 'freed up'. Otherwise, I would look to see if the retaining pin is sliding smoothly in the thick pad pin hole. Maybe clean up the pin a little and grease it. Deburr it if nessessary.

[Richard UK]

It's worth pulling the pad pins (front and back), cleaning off any corrosion and putting a smear of 'coppaslip' on them once a year.  If you don't, there's a danger that they will seize in there with corrosion and make pad changing almost impossible.

[BRB96Z34]

Thanks for the help, I guess it was only stuck.  Once I got the pads off the pistons moved freely.  They now move together.  My next problem is the outer pad.  Do I replace only the one or both as a set.  The outer is almost fully worn but the inside has plenty to go.

[scratch]

Always as a set (you can't buy just one side, they sell them as sets anyway). Maybe keep the thick one (and the thin one) as a buffer for pushing pistons in when you work on the rear caliper (like when you use a screwdriver to leverage the piston back in).

[BRB96Z34]

That's a good idea.  Thanks a lot.