Servo problems

OK, I’m having an issue with servo actuators. I have WASD all set up, and a Python script controlling them.

To make the player move left or backwards the script inverts the force values (from 300 to -300 for example), but I’ve noticed that strafing left or backpedalling (where it gets negative force) can bypass the linv limits. I’ve tried inverting the linv also, which makes the character do nothing at all while S or A is pushed, and I’ve tried to invert just the linv (leaving the force positive), which results in the same thing (doing nothing at all).

Does anybody know how to fix this?