I've also had pretty good luck using a small shop vac, configured to blow.

I tape the nozzle to (for example) that drain hole you guys have been discussing.

This pressurizes the mast a bit.

Then I use a bit of water with a bunch of liquid dish soap in it to "soap up" sections of the mast.

The area that is not properly sealed will "blow bubbles" for you, identifying the leak.

You also get a cleaner mast in the bargain.

I like this better than derigging the mast so I can remove it from the boat and transport it to a swimming pool.