I believe you're right tomekcatkins. The french also say 'raining buckets' as opposed to the English 'raining cats and dogs'. Jamiro, for an explanation, go to: joe-ks.com/phrases/phrasesR.htm
Just a general remark: the word ceber is rather obsolete, you won't hear it in everyday's speech. More often you may stumble across it in crosswords.
It only survived in this "leje jak z cebra" expression. Some Poles don't even remember what a ceber was. I suspect, however, that it may still be heard in some rural areas.