How do I get a 'water dispenser' to stop 'dispensing' water when it should not be?

I asked this earlier, but perhaps in the incorrect section, so will try again. It is to do with my chooks and a home-made water dispenser - one which self re-fills as the chooks drink the water, so the level remains the same.

I made one ages ago and it has been working beautifully. I have just replaced my original bucket with a larger one, now it keeps running water out of the holes all of the time and I can't figure out why.

My 'design' is simply, a large bucket with a tight fitting lid. This sits inside a large deep sided pot plant base. I drilled a few holes in the lower part of the bucket, below the rim of the pot plant base edge. As the chooks drink the water and the level drops, the now exposed holes then allow water to run out and refill the pot plant base to the brim again. (Water stops running out obviously) This allows my chooks to have fresh water for several days without my having to keep refilling their water dish. Great for when I have to go away for a few days!

I have just today replaced my 10 litre bucket with a 20 litre bucket using the same principal.....except the water just keeps running out all the time - and I can't figure out why! Maybe because am so tired.

Except for the different size bucket, which should not make any difference, the only other difference is that with my original bucket, I tried to soften the base and 'mould' it a bit to fit a bit of a bump in my pot plant base. In doing so, I managed to burn it and in doing so, melted a rather large hole into it. It still worked and was the only bucket I had, so used it - and like I said, it worked brilliantly!Wondering if this gaping hole in the bottom of the bucket is why it worked - and this one today is not?

Any geniuses out there that can offer any suggestions please? I am wondering whether I should drill a couple of big holes into the base of the new bucket to create some sort of airbubble effect or something?

Any assistance and all suggestions will be enormously appreciated! Got me somewhat stumped! It has taken me ages to find a suitable bucket - and they do not come cheap (around $40 - so don't want to be cutting or drilling more holes into it without good reason).

I DO realise I should just do what I did before, but I would like to know WHY it works.....before I stuff up an expensive bucket possibly. Just in case?!............. I cannot re-install the first one - it got damaged.

Many thanks.