Why does a heavily loaded vehicle take an especially long distance to stop once its spring brakes engage from low air pressure?

Explanation

A heavily loaded vehicle takes an especially long distance to stop once its spring brakes engage because spring brakes do not act on all axles, so only part of the vehicle's total braking capacity is working, while the full weight of the load still has to be slowed. 49 CFR 383.111(b)(7)(iv) requires CDL applicants to know about the parking brake as part of general operating practices, and spring brakes are the mechanism behind that parking and emergency brake function; the specific loaded-vehicle stopping-distance consequence is standard driver-training content from the manual, not a figure stated in the CFR paragraph itself. Saying spring brake force weakens as pressure keeps dropping is backwards: the spring pushes harder, not weaker, as the opposing air pressure falls, and the real issue is which axles they act on at all, not any loss of force. Claiming a heavier vehicle's spring brakes engage at a different psi is wrong because the 20 to 45 psi activation range is a function of the valve design, not the vehicle's load. Spring brakes disengaging above a certain weight is wrong and would be dangerous; they are a fail-safe mechanism that engages regardless of how loaded the vehicle is.

Source: 49 CFR 383.111(b)(7)(iv)