N20879DG | Genuine Haldex® Automatic Drain Valve

1. Universal Commercial Vehicle Compatibility

Because heavy-duty pneumatic air brake networks rely on standardized reservoir port sizes, this Haldex automatic drain valve features a universal 1/2" NPT (National Pipe Thread) male mounting port. This allows it to serve as a direct-fit or upgrade replacement across practically all major heavy-duty truck, bus, and trailer platforms, including:

• Semi-Trucks: Freightliner, Kenworth, Peterbilt, International (Navistar), Volvo, Mack, and Western Star.

• Commercial Buses & Coaches: MCI, Prevost, Gillig, and Blue Bird transit/school buses.

• Heavy Duty Trailers: Vocational trailers equipped with standalone air brake distribution systems (Great Dane, Utility, Wabash).

2. System Integration

This valve is installed directly into the lowest port of the vehicle's primary (wet) or secondary (dry) air storage tanks. It utilizes the vehicle's brake application pressure drops (or an auxiliary governor signal line depending on the specific plumbing configuration) to automatically cycle open and "spit" condensed water and compressor oil mist out of the air system before it can travel downstream to corrupt ABS modulators, brake chambers, or leveling valves.

1. What is it?

Commonly known in the heavy-duty trucking industry as a "spitter valve" or moisture ejector, this device is installed at the lowest point of an air brake reservoir tank. It serves as an automated alternative to manual pull-cord drain valves, eliminating the need for drivers or fleet mechanics to manually drain water from air tanks every day.

2. Core Functions

• Automated Moisture & Contaminant Purging: As compressed air cools inside the vehicle's steel or aluminum storage tanks, water vapor condenses into liquid water, and compressor oil mist settles at the bottom. This valve automatically senses pressure drops (such as when the brakes are applied or when the compressor cycles) and momentarily opens to forcefully eject ("spit") these contaminants out of the system.

• Corrosion & Component Protection: By constantly removing water and oil, it prevents liquid from traveling downstream into the brake lines. This protects expensive pneumatic components—such as ABS modulator valves, air dryers, leveling valves, and brake chambers—from internal rust, seal degradation, and winter freeze-ups.

• Integrated Maintenance Features: The N20879DG model features a standard 1/2" NPT inlet mounting thread and includes an internal fine-mesh screen or filter to prevent carbon flakes and debris from jamming the main sealing seat open, ensuring leak-free operation.

1. Constant Air Leaking / Continuous Hissing ("Stuck Open")

The most common sign of failure is a continuous hiss of escaping air from the bottom of the valve when the air tanks are fully charged. This happens when carbon flakes from the air compressor, rust scales, or debris bypass the internal screen and get trapped between the rubber seal and the valve seat, preventing it from closing completely. This creates a parasitic air leak that forces your compressor to work twice as hard.

2. Excessive Water in the Air Tanks During Manual Checks

If you open the manual petcock or pull-cord drain valve on your auxiliary tanks and a large stream of water or oily sludge pours out, the Haldex automatic valve has stopped cycling. When internal springs weaken or the internal pressure-sensing piston gets gummed up with thick oil sludge, it stops opening ("spitting"), allowing moisture to dangerously build up inside the reservoir.

3. Frequent, Erratic, or Loud "Spitting"

While a healthy valve should make a short, sharp "psst" sound when air pressures fluctuate, a faulty unit might begin to cycle rapidly and loudly every few seconds, or it might get stuck in an endless clicking loop. This erratic behavior indicates that the internal diaphragm is ruptured or the control line supplying the pressure signal is blocked or leaking.

4. White or Yellow Sludge Built Up Around the Discharge Port

If you look beneath the truck at the valve's exhaust port and see a heavy, crusty accumulation of thick white aluminum corrosion or milky-yellow oil emulsion, the valve is heavily contaminated. This sludge eventually glues the internal mechanical parts together, causing total component seizure.