Cat 289D brush cutter problems

I have a Cat 289D compact track loader Serial number CAT0289DCTAW00565 when I put on continuous flow to brush cutter the machine will move for a few seconds and then wont move at all until I stop flow to brush cutter. If I only go about half way on the roller knob to limit the continuous flow to half instead of full the machine will move but but brush cutter will bog down when trying to cut since blades aren't rotating at full speed. I'm at a loss tried two different brush cutters and they both do the same thing. they are low flow brush cutters and I even tried running high flow and it makes no difference