Make sure that there isn't a crack in the servo bore allowing it to leak or a problem with the tube from the valve body to the case. If the surface wasn't good on the clutch drum, it could cause it to not hold. I drill holes in the drum under the band to help lubricate it, sounds like yours might already have them.
You didn't state if it was a stock band or an aftermarket. I have heard that you don't want to clean the aftermarket ones in parts cleaner.
The other thing would be that the linkage arm hole matches the shifter being used. I changed from a B&M to a QuarterPro and had to buy the different lever, because I could never get it to pull in 1st correctly after lining it up for neutral.
One of the last things that I can think of would be a problem with the valve body. Some times the valve bodies don't use gaskets at all and they can cross leak. Make sure that if it has the gaskets that they are the correct ones and don't cover any holes in the plate. There are a couple of gaskets that don't cover all of the valve body, so there can be a leak with them.
Just some more thoughts.
Good luck,
Bill