GMC TopKick Warning Lights: What They Mean

Red: stop or act now

Brake System Warning Light

Hydraulic brake fluid in the master cylinder is low or the hydraulic brakes have another problem; a warning tone also sounds.
What to do: If it comes on while driving, pull off and stop carefully. If it stays on, have the vehicle towed for service. PDF page 145

Other warnings and indicators

Air Brake System Warning Light

LOW AIR light: brake reservoir pressure fell below 60 psi. Below 35-45 psi the parking brake may apply suddenly.
What to do: Drive only to the nearest point of safety and stop; stop as soon as you can. PDF page 146

Service Brakes Soon Warning Light

There may be something wrong in the brake system; a warning tone also sounds.
What to do: Have the system repaired immediately. PDF page 145

Parking Brake Warning Light

Comes on when the parking brake is set, and stays on if the parking brake does not release fully. PDF page 146

Antilock Brake System (ABS) Warning Light

Stays on or comes on while driving: the vehicle needs service. Brakes still work but without the antilock feature.
What to do: Have the vehicle serviced. If brake or low air light is also on, the regular brakes have a problem. PDF page 147

Trailer Antilock Brake System Warning Light

If it stays on, there is a problem in the trailer's antilock brake system. It is not functional without an ABS trailer.
What to do: Have the system repaired immediately. PDF page 147

Summarized in our own words from the 2010 GMC TopKick owner's manual (AI-assisted; every light name was matched against its cited page 2026-10-18). Your vehicle may not have every light, and other years can differ. A red warning that stays on usually means stop safely and check: your manual has the final word.