South African Heavy Truck Develops Multi-Wheel Brake Drag After Pedal Release as Air-Brake Relay-Valve Residual Delivery Pressure Is Investigated

Case Background

A heavy diesel truck operating in South African stop-and-go freight service developed repeated rear wheel-end heating.

The driver released the brake pedal normally, but several brake chambers on the same axle group retained low residual pressure.

Because multiple wheel ends were affected together, technicians looked beyond individual brake mechanisms toward a shared pneumatic control component: the relay valve.

Brake Release Requires Air Pressure to Leave the Actuators

A relay valve helps apply and release brake pressure close to the brake chambers.

A simplified sequence is:

Brake Applied

Control pressure arrives → Relay valve opens reservoir air to delivery ports → Brake chambers apply

Brake Released

Control pressure falls → Delivery supply closes → Relay valve exhausts chamber pressure → Brakes release

If the exhaust stage is incomplete:

Pedal released → Control pressure disappears → Delivery pressure remains partly trapped → Brake chambers stay partially applied

Why Multiple Hot Wheels Were an Important Clue

A seized caliper, foundation-brake mechanism or chamber normally affects one wheel or one localized assembly.

When several brakes supplied by the same relay valve show similar residual pressure, the common pneumatic node becomes more relevant.

Possible causes can include:

• contaminated relay spool;
• blocked exhaust port;
• damaged valve seat;
• diaphragm issues;
• freezing or moisture depending on conditions;
• or incorrect valve installation.

South African Stop-and-Go Operation Reproduced the Fault

Frequent brake applications created repeated cycles:

• brake pressure applied;
• truck slowed;
• pedal released;
• relay valve failed to exhaust fully;
• residual chamber pressure remained;
• repeated cycles increased wheel-end temperature.

The symptom became stronger as operating time increased.

The Investigation Measured Pressure During Release

Technicians reviewed:

• control-line pressure;
• relay-valve delivery pressure;
• chamber pressure after pedal release;
• exhaust flow;
• brake-chamber pushrod return;
• and mechanical brake freedom.

The key observation was whether pressure returned to the expected released condition after the control signal disappeared.

Why Mechanical Brake Service Alone Could Fail

Replacing linings, drums or individual chambers does not correct trapped pneumatic pressure generated by a shared relay valve.

The new components may continue dragging.

Difference From an Air-Dryer or Compressor Fault

Air dryers and compressors affect air generation and conditioning.

A relay valve manages delivery and exhaust of brake application pressure.

This case therefore concerns pressure remaining where it should have been released.

Technical Lesson

Air-brake diagnosis should compare:

command state

with

actual actuator pressure.

A released pedal does not prove that every brake chamber has been depressurized.

FAQ

Can one relay valve cause several brakes to drag at the same time?

Yes. Multiple chambers can share the same relay-valve delivery circuit.

Can the brake pedal feel normal?

Yes. The control signal can release correctly while delivery pressure remains trapped downstream.