On the roller brake tester, one wheel on that axle produced noticeably less braking force than the other. It's a measurement, not a diagnosis — it says the two sides aren't matching, but it doesn't say why. In real driving this is what makes a car pull to one side under braking.
Wording as it commonly appears on an MOT certificate: “Braking effort imbalanced across an axle”.
What does it mean?
On the roller brake tester, one wheel on that axle produced noticeably less braking force than the other. It's a measurement, not a diagnosis — it says the two sides aren't matching, but it doesn't say why. In real driving this is what makes a car pull to one side under braking.
How serious is it?
Usually recorded as: Major, Dangerous. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- A sticking or seized caliper on the weak side, so the pad isn't being pressed properly
- Contaminated pads — oil, grease or brake fluid on the friction surface
- A collapsed flexible brake hose acting as a one-way valve
- Glazed pads or a corroded disc surface on a car that's been standing
- On the rear, a seized handbrake cable or worn shoes on one side only
What a mechanic would check
Wheels off both sides of that axle and compare. I'm looking for uneven pad wear side to side, whether the caliper pistons wind or push back freely, whether the slide pins move, and whether the flexi hose is soft or bulging. A quick road test with a hand on each wheel afterwards tells you which side is running hot.
What might need replacing
- Caliper overhaul kit, or a replacement caliper
- Pads on that axle if they're contaminated or glazed
- A flexible brake hose
- Handbrake cable or rear shoes on the affected side
Can you still drive it?
Depending on how big the imbalance is, this can be recorded as Major or Dangerous. A car that pulls hard to one side under emergency braking is a genuine hazard, particularly in the wet, so treat it as urgent and get it looked at before driving any distance.
Repair cost
Completely depends on which of the causes above it turns out to be — a seized slide pin cleaned up is a world away from a new caliper. This is exactly the case where you want the fault confirmed before anyone orders parts.
Before the retest
Brake work is usually done in axle pairs — pads and discs are replaced on both sides of the same axle so the car pulls up straight on the roller tester. After new pads or discs, drive gently for the first hundred miles or so if you can, because brakes need bedding in before they reach full efficiency. Keep the old parts or the invoice; a tester will not ask for them, but they settle any argument about what was actually replaced.
Retest rules
If the vehicle stays at the test centre for repair and is retested within 10 working days, a partial retest is often free; some items qualify for a free retest if you bring the car back by the end of the next working day. The exact conditions are set out on GOV.UK, so confirm with the test centre before you leave. A major or dangerous failure means the car cannot legally be driven away unless the previous MOT is still valid and nothing dangerous was recorded — check your certificate first.
Related help
More brakes failures · MOT failure decoder · Ask a mechanic about your certificate
This is general guidance from a UK mechanic, not a substitute for a physical inspection. Only the tester who inspected your vehicle can confirm what was recorded and why.