Had a really crusty one the other day. Used an  old line fitting, drilled and tapped the inside to take a standard grease fitting. Closed off the bleed nipple, attached my grease gun to the fitting and pumped away. Had to clean up some grease, but the piston slid right out.

MUCH safer than compressed air.....

Using compressed air creates a flying projectile so it should be aimed into a partially full garbage can. It is strangely satisfying when it lets go!