I have not started nor driven my truck for about 6 months.  She started right up but the exhaust brake in not working.  I have suspicions that our dumbassed cat which has been crawling up into the engine compartment may have something to do with it.  I think I have some ideas on where to start troubleshooting but it never hurts to get as much information and advice as possible before hand.  Does anyone have any comments or suggestions on solving this problem.  Thank!  

If it uses vacuum I'd start there, or a lose wire maybe 

Mine uses vacuum and and it failed in the actuator or ar least I think it has. I have been thinking of late that it just could be some worn fittings that might not be letting enough vacuum get to the actuator. 

Check your vacuum source like the others have mentioned. Then double check the wiring to the vacuum solenoid valve. Have someone toggle the switch on and off and check for the vacuum on the other side of the solenoid valve. Like my Jacobs Brake, there is little plastic pins that lift the valve and they wore down so I made need ones out of wire coat hanger. Repaired my solenoid valve. Basically, if there vacuum in the line going to the valve then there might be an issue with the diaphragm in the exhaust brake.