I recently rebuilt my Berkely 12JC-A and had it out for the first time yesterday.  It worked great in forward and neutral but when reversing the bucket there was a noticable RPM drop and it felt like the motor/pump was loading up pretty good.  Is this normal when in rev. or is there something going on that needs attention?

With the exception of a combination Hand throttle/Fwd Reverse  controller , when your reversing the jet your simply redirecting the water flow the engine doesn't know the difference How is your jet and throttle controlled ?

Any opinions on this?

Typically theres Zero difference in engine load *(at idle) between FWD and Reverse

That's what I had thought, but once the bucket is closed for reverse the engine rpm noticeably drops after about a second or two.  I'm not sure if I did something wrong in the rebuild but the pump is pretty straight forward when assembling, would it need to break in the packing/bearing?  I did use grease in the bowl bearings instead of gear oil but figured there were many other people running the same way.

How many RPM does it drop?

It would drop around 200-250 rpm (just an estimate but very noticeable when it happens), this is just from idle which is around 850 rpm.