So I'm backing the trailer up yesterday, and as I'm looking in the side mirror I see the trailer tyre deflate instantly! Not just a nail in the tyre or anything, but 2 secs to dead flat kind of deflation. I thought I must've run over a blade or something, but there didn't seem to be anything on the ground or visibly sticking out of the tyre.

However the brake caliper had lost a retaining bolt and had rotated outwards towards the wheel. It swung back into position very easily, so I wasn't worried about it.

I took the wheel off today to look for the puncture. Nothing in the tyre. I stuck the air hose on, and realised air was pissing out somewhere! Then I turned the wheel over.

And here's the problem. Reckon it's still good??: