which engine? i have the 2200, when i changed mine what i had to do was unbolt the pump, pull the pulley off, then pull the pump/resivoir out, then to reinstall i took the resivoir off, put the pulley back on, set it back down in there (where it gets mounted) then i put the resivoir back on, it was a complete pain in the ass but its the only way i could do it,
but to answer your question, no, i seriuosly dont think you'll be able to pull the resivoir off without pulling the pump out, if you do try to pull the resivoir off without the pump, you will need to unbolt the pump anyways
edit: now that i think about it, the o-ring you'd replace by pulling the resivoir off would be the suction port o-ring, theres absolutely no pressure behind that one so theres a good chance thats not where your leak is...