Retracting brake pistons with callipers in situ

Hi, As my Hijet failed MOT with one partly stuck on front brake and overall too low efficiency I've been messing about them trying to retract the pistons and getting more movement in the brakes.

I figure if I can retract the brake pistons a few times and get the calipers moving about on their mounts then I can get a little grease in and generally have things working more smoothly.

With a g clamp on the inside edge of the caliper to the back of the outer brake pad I have retracted the piston a few mm and loosened things up a bit but I'd really like to fully retract the piston.

Anyone managed to do this with the calliper in situ?

Thnaks

If its anything like mine the sliders will be gummed up just take the 2 bolts out smash them out clean em up bit of lube and back together ,Bish bosh