A tag axle operating system comprises a gas circuit with gas at a prescribed pressure and a hydraulic circuit that delivers hydraulic fluid pressure to raise and establish the tag axle in an inactive condition and to lower and establish the tag axle in an active condition. The hydraulic circuit establishes a prescribed hydraulic pressure opposing the gas pressure and a resulting force forcing the tag axle tires to bear against a road surface and the tag axle to accept a predetermined load. A brake valve allows air pressure delivery to operate tag axle air brakes while the prescribed hydraulic pressure is maintained and prevents such delivery to disable the brakes when the prescribed hydraulic pressure is relieved. And the hydraulic circuit operates to relieve the prescribed hydraulic pressure to relieve the force on the tag axle and disable the brakes when the tag axle experiences a predetermined tilt angle.