Stand it in the sun with the extension removed & arms on the ground, head against the wall or whatever.
Make sure it is at about 45 degrees so you get a lot of heat on it, if it's not black wrap in black plastic.
The air inside will expand and push the water past the plugs.

Same happens in reverse when you get in the water with a hot boom, it sucks the water in.

Just remove the plugs and leave them out

Alex_sc said..
Mmm not sure how you get the plugs out though?? The plugs are in about 80cm from the boom end.

You can sometimes blow them out with compressed air but doesn't always work.
Sometimes they are just foam & blow through easy but if they are rubber they might not.
Seal the air nozzle in the end of the arm & give it all you got. (wear safety glasses) and stay clear of the other arm so you don't get shot

The other way is to attach a big self tapping screw to the end of a long dowel or steel rod, poke it through the plug & try to pull them out, also doesn't always work and now they have holes in them

If you stick a dowel up the arm and give it a good push, you can tell if they are tight or not, if loose give it a go.