Skid Steer mid throttle trim always powers up at 1000pwm

Trying to get a skid steer vehicle working. Im using the right TX stick center sprung for FWD/REV and steer. RC inputs working correctly.
When I set Servo 1 & 3 trim to 1500 the output does what its supposed to but at every power up it defaults to 1000.
Original pixhawk running 4.1.5
Any ideas please?