According to this article the rear are spring loaded and the front are not. Bold and Underline are me helping to point to relevant info.
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The parking valve is typically located on the dash. With normal system pressure, pushing the valve “in” causes air to flow to the parking (emergency) side of the double air chamber, retracting the push rod against its applying spring and releasing the brakes. The parking/emergency system is spring applied and air released. The push/pull dash parking valve is unique in that once the supply pressure drops to the valve’s internal setting (generally between 20 and 45 psi), the valve will automatically pop out, thus fully applying the rear spring brakes. In an emergency, the parking valve can be manually pulled out at any time, and rear braking will be experienced. In addition, the parking valve may be manually pulled out at any time in an emergency, and again, braking will be applied to the rear brakes.
Understanding that the parking/emergency brake system uses the bus’ rear brakes and that it takes air pressure to release them explains why a bus that has not been used for an extended period of time and has lost air pressure will have heavy brake drag until there is sufficient air pressure buildup to cause the rear brakes to fully release. Always wait until the warning buzzer is off before moving. All buses built in 1975 and thereafter have a dual brake system. One system operates the front brakes, while the other system operates the rear brakes.
These systems can be readily identified by the presence of a single gauge with two needles. (Two single-needle gauges are also used.) In dual systems, should air be lost in the front system, the rear braking system will stop the bus. Conversely, should the rear brake air supply be lost, through special valving, the rear spring brakes’ release air will be bled off in direct proportion to the front brake application air pressure, thus stopping the bus normally. In either of the above cases, the low-air warning systems will be activated due to low air pressure in the failed system; however, the bus can be safely pulled to the side of the road and stopped.