A physical break in the control air tubing -- disconnected, cracked, or kinked. Simple problem with dramatic results: the entire pneumatic control system stops working. Common after maintenance, rough transport, or vibration over time.

What you'll see

The compressor was working fine, then suddenly stops making pressure. Or it stopped working after transport or maintenance. The engine may run but at the wrong speed (idle or maximum, depending on where the break is). If the break is on the line feeding the engine speed actuator, the engine stays at idle. If the break is on the line feeding the inlet valve, the valve stays in its default position (usually closed). You may hear a hissing sound from compressed air leaking through the crack.
Before you assume this is the problem

If all control air lines look intact, the problem may be in the regulating valve itself or in the actuators. See: Regulating Valve Failure, No Control Air.

See all causes of not building pressure / won't load →

How to diagnose

  1. Visual inspection of all control air tubing

    Trace every control air line from the regulating valve to its endpoint (inlet valve actuator, engine speed actuator, blow-down valve, etc.). Look for disconnected tubes at push-in fittings, cracked or hardened tubes, kinks, and tubes that have vibrated against sharp edges. The small-diameter nylon tubes are vulnerable to damage and UV degradation.
    Result: Damaged or disconnected tube found = fix it.

  2. Listen for air leaks

    With the compressor running (if possible), listen for hissing sounds around the control air lines and connections. Even a small crack will hiss audibly. If you cannot run the compressor, pressurize the control system manually with a hand pump or regulated air supply and listen.
    Result: Hissing = leak location identified.

How to fix it

  1. Replace damaged tubing

    Replace cracked, hardened, or kinked tubes with new tubing of the same diameter and material. Route the new tube away from hot surfaces and sharp edges. Secure with cable ties every 200-300mm to prevent vibration damage. Push-in fittings make this a quick repair -- no special tools needed. Always carry spare tubing and fittings on site.

Common mistakes

Do not use the wrong diameter tubing. The control system response depends on the correct air volume and flow rate through the tubing. Too large a diameter makes the system sluggish. Too small restricts flow and can prevent the actuators from moving fully. Check the existing tubing size before buying replacement. Common sizes are 4mm, 6mm, and 8mm outside diameter.

Parts & tools

Replacement nylon or polyurethane tubing (correct diameter). Push-in fittings. Cable ties for securing. Tube cutter or sharp knife. No special tools needed.

Review safety precautions before starting →

This issue can also cause