What does it mean if the spring brakes apply automatically while you are driving?

Automatic application of the spring brakes during driving means air pressure has dropped below 20–45 psi, which is well below the safe operating range. This is a serious emergency: the parking brake is now applied at full force, which can lock the rear wheels and cause a skid or jackknife. The cause is almost always a major air leak or compressor failure. As soon as you sense the spring brakes applying — usually you will feel sudden deceleration and the low-air warning will have been going off — get the vehicle off the road as safely as possible and do not attempt to release the parking brake by pumping air back into the system without finding and fixing the leak. Continuing to drive after a fault this serious is dangerous and illegal.