My scanner bar started stuttering and getting stuck. The easiest way to repair this would be to replace the scanner assembly, as taking the scanner assembly apart does not appear to be covered by the repair manual.
I was able to take the thing apart, take apart the scanner, reseat things, and put it back together, and now the scanner works without getting stuck or making noise. HOWEVER there is probably A LOT of potential for messing this procedure up, and there are parts that are very finicky.
So if you want to try to fix this, the first thing you're going to need is this file -- the service manual: http://support.brother-usa.com/Virdata/C... SM_MFC_J280W_J425W_J430w_J435W.pdf
Follow the instructions from page 3-49 (PDF page 186) through 3-53 (190 in the PDF). Now you should have your scanner unit detached from the rest of the printer. And now you enter no man's land - the manual doesn't seem to help with the rest.
Basically next I took out all the screws I could see. After that, I was able to get the glass to separate from the lower part of the scanner. Under the glass is the scanner bar, the motor that drives the belt that moves the bar, and TWO WEIRD WHEELY THINGS. In my case, I think one of the wheely things was out of place and causing problems. To get the wheely things in place, turn the scanner assembly right side up and look at the top and bottom of the bar. The wheely things are shaped slightly differently and will only mate with one end of the bar each. Before putting them on, move the bar all the way to the left and make sure the cable at the top that follows under it and gets pushed back when the scanner returns to it's home position is smooth and not kinked if you can. Then place the wheely things in position and CAREFULLY replace the glass, ensuring that the wheely things don't move out of place. After this, put all those screws you took out after departing from the manual back in ASAP so that the wheely things lock in place. Now, put the thing back together and pray to your favorite deity. In my case, this worked out.
Worst-case Ontario, you'll just have to replace the scanner assembly (basically the entire upper portion of the printer that is hinged).
Good luck. You'll need it.