CM 870 Engine Brake

[Nostalgic]

Could be a lot of things. Bad connection in jake harness, poor contacts or bad solenoid, lack of oil pressure due to worn shafts or jake rocker bushing, bad o-ring in solenoid, etc.

You can check with the cover off at idle, look for oil gushing (or lack of oil) and test the solenoid by pushing the button down and see if you have an oil issue at the solenoid itself.