What will cause excessive air pressure drop during a brake application?

Excessive pushrod travel is a key factor that can lead to an excessive air pressure drop during a brake application. When the pushrod travel is greater than the designed specifications, it means the brake shoes are moving further away from the drum than necessary. This increased travel requires more air to engage the brakes fully, which can lead to a significant drop in air pressure.

Additionally, an excessive pushrod travel may indicate that the brakes are not properly adjusted or may have worn components that affect the overall effectiveness of the brake system. This misalignment can exacerbate air pressure drop because the air brake system not only relies on pressure to apply the brakes but also on the correct positioning and movement of the brake components.

In contrast, other potential causes like brakes adjusted too tightly would generally lead to brake dragging rather than an excessive air pressure drop. The air governor cut-out set too low would cause the system to not reach optimal pressure, but it doesn't directly relate to air loss during braking. A leaking service tank relief valve would cause air loss but would not specifically affect the amount of air required during a brake application. Thus, excessive pushrod travel is the most direct contributor to an excessive drop in air pressure during braking.