Hope mono mini problem

My rear brake has developed a problem, it used to be that I could leave the bike for ages and the brakes would be in fine working order all the time.
I got my bike out today to give it a bit of TLC after neglecting it for the road bike for a while and I found that the rear caliper was stuck on (i.e. the pads were not retracting).
I took the wheel out, pushed the pads back manually and returned the wheel, so far so good wheel turning nicely with no rubbing from the disc rotor.
I then tried applying the brakes, stopped nicely, same amount of power as before but the pads did not retract so the brake is "dragging" a bit. There is also hardly any travel in the brake lever.
This happened before and I think it may have been due to moisture in the dot fluid.
Is there a cure/preventative measure or will I just need to bleed the brakes and replace the fluid every year?
Would changing to braided hoses help? is there too much fluid on board?
Thanks in advance.
I got my bike out today to give it a bit of TLC after neglecting it for the road bike for a while and I found that the rear caliper was stuck on (i.e. the pads were not retracting).
I took the wheel out, pushed the pads back manually and returned the wheel, so far so good wheel turning nicely with no rubbing from the disc rotor.
I then tried applying the brakes, stopped nicely, same amount of power as before but the pads did not retract so the brake is "dragging" a bit. There is also hardly any travel in the brake lever.
This happened before and I think it may have been due to moisture in the dot fluid.
Is there a cure/preventative measure or will I just need to bleed the brakes and replace the fluid every year?
Would changing to braided hoses help? is there too much fluid on board?
Thanks in advance.
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Hopes are a doddle to bleed anyway
Cube LTD 29er
Glentress Trailfairies
Have now replaced the piston seals but the pistons still don't retract fully. The old pistons appear to have a roughened surface where they would stick out from the caliper. So, new pistons it is too! Will post another update when the new pistons have been put in.
PS. parts for Mono Minis are quite hard to find these days. I bought mine at ukbikestore.co.uk whose service is pretty good.