On a vehicle equipped with an application pressure gauge, what does it mean if you notice you need increasing application pressure to hold the same speed down a grade?

AAMVA CDL Manual, Section 5.1.10 (Application Pressure Gauge), p. 5-3; 49 CFR 383.111(b)(7)(i) (proper braking techniques)

Brake fade on a long downgrade is a leading cause of runaway truck incidents. Recognizing rising application pressure as an early warning sign lets a driver downshift and slow down before losing effective braking entirely.