Air is the easiest way, but how much pressure are you using ?? You need to regulate the air down to under 5 lbs..2-3 is all you need to balloon the seal, once you have positioned the piston JUST right on it, anymore pressure than that, and you are fighting it to push the piston in. Also, as soon as the seal balloons around the piston, shut the air off.
Have a film of brake fluid on the o-ring to help the piston slide in.

Just out of pur curiosity does anyone make a stainless steel bleeder ?

As a follow up, my friend who ownes a small auto repair shop and his very experienced mechanic did the air/balloon method for me,
although it wasnt easy....took maybe five minutes with one guy controling the air blower and the ace mechanic manipulated the piston
until he could get the dust seal rubber to come around the edge of piston....He was using high air pressure and wouldnt listen to me about
trying low pressure...............