the very same thing used to happen on my 7.5t lorry that i used to drive for a company. i used to keep a hammer and would often give the exhaust brake a tap if it got stuck on. the other option is not to use the exhaust brake atall, just leave it turned off. or if it bothers you that much just dont drive it and tell the company why. aslong as you fill in a defect report the blame falls onto them for not fixing the problem, i cant see you crashing the lorry because of that problem anyway. its a minor issue in my opinion.