HOWO Air Brakes Losing Pressure? Drain, Test and Find the Leak

Air brakes are a pressure system

On a HOWO, the brakes work with air, not fluid. A compressor fills the tanks, the air waits there under pressure, and every press of the pedal spends a little of it. That is why an air brake problem usually starts small: the needle sits a bit low, the compressor runs longer, the buzzer comes on a little earlier each week. Find the leak while it is small and the fix is cheap. Wait, and the truck parks itself on the roadside.

What the two gauges tell you

Most HOWO trucks carry two needles, one for each brake circuit. After start-up they should climb together to the normal working band, around 8 bar (about 120 psi). While you drive, the needles should sit still. A needle that slowly falls while you roll is the first sign of a leak. If the low-air buzzer sounds or the red lamp comes on, stop in a safe place — below that line the brakes get weak fast.

Why there is water in the tanks

Air always carries a little moisture, and compressing it squeezes that water out. The compressor also passes a thin oil mist into the system. Both land in the tanks, where they rust the metal and gum up the valves. Humid coasts and dusty roads make it worse.

The free daily habit: drain the tanks

Open each tank drain valve until the air runs clean and dry, then rebuild pressure before you move. It costs nothing and it protects every valve behind the tank. A drain valve that will not seal, or has seized shut, is a cheap part — see the drain valve WG9000361403.

The 10-minute leak test

Build full pressure, switch the engine off, release the parking brake and listen. A small drop in the first minute is normal; if you can watch the needle move, you have a leak worth finding. Brush soapy water over the fittings and valve bodies — bubbles mark the spot.

The five places air escapes

1. The compressor and its feed line. A worn compressor builds pressure slowly and passes more oil. If the gauge takes much longer than usual to reach the normal band in the morning, check it before you blame the valves — see the air compressor 202V54100-7134/1 and the compressor WG1500139006.
2. The air dryer cartridge. The dryer takes the water out. When its cartridge is old, water walks straight into the tanks and you will see it at every drain. Change it with the filters — our dusty-road schedule covers the rhythm.
3. The drain valves. Grit holds them open a crack; rust welds them shut.
4. The valve bodies. Worn seals inside a hand brake valve, an ABS magnetic valve or an air control valve leak air out of the exhaust port — the hiss you hear at the valve, not at a pipe.
5. Hoses, fittings and couplings. Cracked rubber, loose clamps and worn couplings leak under load, when the air is hot and moving.

Fix the cheap parts first

Drain the tanks, find the hiss, and only then buy parts — most slow leaks end at a drain valve, a fitting or a cartridge, not at the axle. While you are under the truck, look at the brake pad assembly WG9100443050 too, and keep a spare valve or two on the shelf — a solenoid valve WG9719710004 weighs almost nothing. If you run long routes, a small box of spares in the cab pays for itself: see our roadside spare parts kit. For the full range, our brake system category has the valves and compressors in one place.

Send us the part numbers from your truck through our inquiry page. Valves, compressors and drain valves ship in small boxes, and we confirm every number before we quote.