Well, if you did it that way and you took the cap off while the pump was running, you'd get water all over the place
Sorry, was kinda tired when I wrote that
So, that configuration would be ok, just dont' have the pump running when you have the cap off. You can also have one on the return, and if it's right above the resivior (or right before the inlet to the pump if it's an inline pump), you'll probably be ok with leaving it off while the pump is on... because there will not be any flow restrictions to the pump, probably.
Good luck! Be careful!
[This message has been edited by Rich W (edited 02-07-2001).]