Hand Vacuum Pumps
This is interesting. I haven't tried building one, and don't know if you could use it to pump into a bucket for hand dredging, but the instructions are good. The author claims he can do it for less than $20 by using a bicycle pump.
http://www.instructables.com/id/make-a-manual-vacuum-pump-for-under-$20-by-convert/
http://www.instructables.com/id/make-a-manual-vacuum-pump-for-under-$20-by-convert/
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