Dont use wd40 as it might damage the seals on the pistions and that would be terminal as new seals aren't easy to get.
I would hold one position in the bore and pump the lever to get the other on out as far as you dare. Then clean it with lint free rag , and lube it with mineral brake oil. Push it back into the bore and repeat , after a couple of cycles it should free up. If it does repeat on the other piston .
If this doesn't work then a strip will be required I'm afraid