Cat E240 track brake sets up while tramming

The right track brake sets up while tracking (either direction) after oil warms up. I have had the Cat mechanic test pressure of the brake valve on the track motor. The pressure drops from 4000psi to 150 psi and of course the brake sets up. If I let up on the left track lever and push again on both levers the brake releases and will track for a short distance and then set up again. The Cat mechanic has torn the brake valve apart and found nothing obviously wrong. I keep saying rotary valve but he and the other Cat shop mechanics want to take the motor off and send it off for testing. I can't understand why the motor would cause the pressure drop when it is working fine otherwise. Any ideas?:Banghead