It should not be normal. I can be at 10 in.Hg for 5 minutes without the vaccum pressure dropping like
@strokeking are saying. Sorry to say it, but if the vaccum pressure can't be kept stable there is a leak in the system.
There is also a factor of volume. If you have a big water trap, like over 2 liter you need to pump a lot in the beginning, in order to get up the vaccum pressure to a reasonable level. But once you are there the vaccum pressure should not drop at all.
If the vaccum pressure drops there is a leak.
We can talk about where this leak can be at. It's probably just something easy. Tightening a fitting or something like that.