Agree,
inside of hoses might be swollen and not allowing fluid to run back.
Take them off and check they are not blocked.
If that doesn't work unbolt master cylinder from the brake booster and see if that releases the calipers"leave the lines connected when you do this"
If that test releases the calipers replace the brake booster.
You might want to do my second suggestion first,"it's easier".
Mark Australia
November 2011