From the Jacobs Exhaust Brakes Owners Manual:
"The Jacobs Exhaust Brake is a retarding device intended to help control vehicle speed. It is not a vehicle stopping device. On an uphill climb with the Jacobs Exhaust Brake ON, taking your foot off the accelerator will result in rapid vehicle slow down due to uphill grade and retarding power. Operation on wet, slick or icy roads is not recommended. Use the Jacobs Exhaust Brake ONLY when you have good, dry traction with the road. Before shutting off your engine, always turn your Jacobs Exhaust Brake dash switch OFF. Your Jacobs Exhaust Brake should also be turned off when the engine is left idling for an extended period of time (longer than 3 minutes).
Never operate the Jacobs Exhaust Brake on wet or icy road surfaces. Always be sure of your traction".