Use something with better anti-corrosion abilities than brake-cleaner (which just dissolves light oil). PB-Blaster will remove rust and corrosion from piston. Squirt some in there and piston will move freely after soaking overnight.
When I managed bike-shop during university, we had lots of bikes come in with stuck seatposts. They'd be mangled and scared-up from endless twisting trying to free them up. We would just spray PB-blaster on them and let sit overnight. By next morning, 1/2 of them would have seats completely lowered by themselves after the PB-Blaster dissolved corrosion from seatpost.
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