I have heard that that is an FSX problem. This is why when using the CIVA-INS you must not Align it with the engines running, it will inch forward.

You could try increasing the brake strength by editing the aircraft.cfg under [brakes]:

[brakes]
toe_brakes_scale=0.85 <- change this to 1.0 to see if it helps
parking_brake=1
hydraulic_system_scalar = 1
differential_braking_scale = 1

From FSX SDK help file (fsxsdk.chm): toe_brakes_scale - Sets the scaling of the braking effectiveness. 1.0 is the default. 0.0 scales the brakes to no effectiveness.

Mark