Brakes for a physics vehicle?

I have a multi-wheeled vehicle created using physics constraints. I need to add friction to the constraints at the wheels to simulate braking. I’ve tried setting the angular motor velocity to zero and the strength to something really high, but the vehicle still creeps on hills. Is there a way to stop the wheel rotation? I also tried locking the axis constraint. But that causes the wheels to spin violently back to zero rotation, flinging the vehicle. This has me stumped. Ideas?