JAKE BRAKE INFO
If you Jake Brake suddenly stops working, then chances are that you have blown the stock 5 amp fuse.   To locate the fuse, open the front hood.   Locate the main fuse box (which is on the driver's side, about eye level) and remove the cover.   As you view the fuse panel, 2nd row down from top and 3rd position from left, there is an
external wire coming out with a seperate fuse holder in the line.  This is the 5 amp fuse for the Jake brake.   If it is blown, I understand that it is permissible to replace it with a 7.5 amp or 10 amp fuse.
At the Freightliner Rally in Las Vegas (March, 2000), it was recommended that you spray the arm of the Jake Brake with silicone spray lubricant every month. Just be sure you do this when you engine is COLD and also wiggle the butterfly to distribute the lubricant. This is especially important if you do not use your coach and exhaust brake regularly.
Does your Jake Brake seem like it just doesn't have the "braking power" like it used to? Well, I had the same feeling, but it was oh so subtle.   Then I found out that three exhaust gaskets were blown.   They probably did not all go at once, so this is probably why I did not notice a bigger affect.