"Stock 240z master cylinders have a residual pressure valve in the rear circuit. This was installed to keep slight pressure against the drum brakes at all times. When you do a rear disk brake swap, the residual pressure valve must be removed, or you will have the slight dragging that you are describing. The valve is in the assembly under the rear reservoir.
Here is a picture from zcarnut's 1" master cylinder thread. The ones in the stock 240z master cylinder look similar and are in the same place, but there is only one in the rear circuit, not the front."
"P.S: You may want to think about upgrading to the 1" master or at least a 15/16" master. I have 280zx rear disks on my 240z which are a pretty similar piston size to the maxima's you have, and with the stock 7/8" master the brakes are very "squishy". I liked the firm pedal feel that the drums had, but the disks stop much better now that I have dialed them in with a prop valve. A larger master would bring back the firm pedal feel that the drums had I think. "