It sounds like you have the stuck faders issue going on. There's a thread in this forum about this issue.
The easiest soultion; There's a thing called the 'Ribbon cable connection correction procedure', where you check that all your ribbon cables are placed right. A slightly loose contact may cause problems for you. Read more about this in the database:
http://www.sonido-7.com/d8b/maintenance.html#Post9The not so easy solution, and my guess: If all faders (17-24 + master) force down and will only let loose if you pull them back and forth some times, but then without a working motor, you have to either replace a chip on the fader board, or replace the whole board. There is three fader boards in the d8b, one for faders 1 through 8, one for 9 through 16, and then the 17 through 24 + master board. The chip for the third board is known as AD8804, and is hidden under the 3rd or 4th fader of the board. It is kinda hard to do yourself, so it should be done by professionals. The chip itself costs only some USD, but the job might cost you. As Mackie doesn't sell these anymore, you'll have to find a used one. On Ebay these costs about 250 USD. Check out the trading-thread here and see if you can get a better deal:
http://www.d8bforum.com/phpBB3/viewtopic.php?f=2&t=240