Repairing eng brake

[mpreiss321]

2011 Int'l prostar Cummins cm871 engine w/685936 miles. I'm about to replace an injector because 1 possibly more have gone bad. I also want to fix an eng. brake issue at the same time. Ok here's the issue- most of the time it works at 1/2 braking power when used in 9th or 10th gear (dash switch on eng 3 setting) While only getting 1/2 braking power I'm hearing a loud squel ( think AC belt pulley seizing up squeal ). That sound will disappear if I change the engine brake switch to eng.2 or eng.1 ( but I would still have 1/2 effective power ). Again truck is traveling in 9 or 10th gear. Engine brake seams to operate fine in lower gears ( all 3 settings). What can be the problem? And how do I fix it?