Draining & refilling Primatic Cylinder CH system

I've got an primatic cylinder CH system and need to drain it to maintain some radiators, is there a special way to do this in order to retain the air lock in the cylinder once it's re-filled?
I intend to get rid of it later in the year and want to do this as cheaply as possible for now.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.