pistons won't come out

Hi there,
I have a ford econoline E350, it has dual pistons on the brake calipers one comes out, when you use the compressed air as it says in the book but the seconed will not any ideas please?
from stephanie

Place a block of wood between the caliper casing and against the one that DOES come out, then apply air to the caliper port - this should force the stuck piston out.

Be careful though, it may FLY OUT suddenly.