You said it yourself Xivex, you don't have a clamp at the pump intake. Your hose might be on tight, but it doesn't mean that it won't allow air to be sucked in.
The pump may not leak while you're using the bucket simply because there is very little flow resistance (i.e. the water is free to flow in and out). Once you close the system, the pump is then subjected to the full resistance of flow, hence your leak. Honestly, I'm surprised that you didn't spring a water leak, but hey, it's still early...
I'd try a hose clamp if I were you. You'll probably be surprised. Also, if you are closing your fill tube, try leaving it open.
If that doesn't work, then look at the pump (yes, I know its an Eheim 1250) but take a very close look for leaks at that fitting (which is removable btw). Use teflon if you haven't already.
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