The air brake desiccant cartridge is plugged or saturated with contaminants.

Replace the desiccant cartridge.

The internal check valve is faulty.

1. Remove the check valve from the assembly from the end cover.
2. Apply air pressure to the delivery side of the valve.
3. Apply a soap solution at the opposite end.
4. Inspect for leakage. A permissible leakage is a 25 mm (1 in) bubble in five seconds. If the leakage is excessive, replace the check valve.

The fittings or the tubing between the air brake dryer and the supply reservoir are faulty.

Determine if air is reaching the supply reservoir. Inspect the reservoir for kinked tubing or for restricted tube fittings.