Parking Brake Inoperative on One Side: MOT Fail Explained
The handbrake still holds the car on the drive, so this one catches people out. The MOT’s roller brake tester measures each wheel separately, and it does not care that the car stays put on a gentle slope:
Parking brake inoperative on one side (1.4.1 (a))
In short: If the parking brake does not work on one side, the car fails with a Major defect under 1.4.1 (a). Separately, the parking brake as a whole must reach at least 16% efficiency (1.4.2): below that is Major, and below half of the required figure is Dangerous.
How the Tester Finds It
On the rollers, each wheel of the axle with the parking brake is driven while the brake is applied, and the machine records the braking force on each side. A healthy system shows force on both. If one side shows nothing, or the car is tested on the road and swerves excessively when the parking brake is used, the manual’s wording is parking brake inoperative on one side, a Major defect.
That is a different defect to the efficiency figure. Efficiency compares the total parking brake force with the weight of the vehicle, and for most cars the minimum is 16%. A car can fail on efficiency with both sides working weakly, or fail on one side while the other side alone almost carries it. Our earlier post on parking brake efficiency below requirements covers that side of things.
Why One Side Stops Working
- A seized or stretched cable on one side: very common on older cars, where the cable rusts inside its outer sleeve.
- A seized handbrake mechanism in a rear caliper, often on cars that sit for long spells.
- On drum brakes, shoes that have worn or a self-adjuster that has seized, so the lever runs out of travel before that shoe touches the drum.
- Contaminated linings, for example from a leaking wheel cylinder.
- On cars with an electronic parking brake, a fault with the motor or wiring at one caliper. The dashboard usually tells you before the MOT does.
Rear brakes do far less work than the fronts, which sounds like good news but is not: lightly used brakes corrode instead of wearing, and a parking brake that is only ever pulled on a flat drive never gets the exercise that keeps it free.
Is It Safe to Drive?
The parking brake is your backup if the main brakes fail, and it is the only thing holding the car on a hill. With one side not working, the car may roll on a steeper slope than your drive, and an uneven parking brake can pull the car to one side if you ever have to use it in an emergency. Leave it in gear (or in park) and get it booked in.
Booked In Soon?
Failed on the parking brake, or noticed the lever coming up further than it used to? Book a free brake check. Any of our 12 branches across Nottinghamshire can take a look, and with over 20,000 MOT tests a year behind us there is not much we have not seen. We will show you what we found, explain it in plain English and confirm the price before any work starts. Book an MOT online when you are ready.
A rear drum brake with the drum off: corrosion in the shoes and handbrake mechanism is a common reason one side stops holding.
Frequently asked questions
Is parking brake inoperative on one side a fail?
Yes. It is a Major defect under 1.4.1 (a), so the car fails and needs repair and a retest. It applies whether the tester finds it on the roller brake tester or, on a road test, the car swerves excessively when the parking brake is applied.
What is the minimum parking brake efficiency for an MOT?
For most vehicles it is 16% (1.4.2). Below the minimum is a Major defect. Below half of the required value is Dangerous.
My handbrake holds the car, so how can it fail?
Holding the car on a gentle slope takes very little force. The roller brake tester measures each side separately, so it picks up a side that has stopped working even when the other side is doing enough to stop the car rolling on your drive.
Does this apply to electronic parking brakes?
Yes. Electronic parking brakes are tested the same way, and a motor or wiring fault at one caliper shows up as a side with no braking force.
How much does it cost to fix a parking brake that is not working on one side?
It depends on the underlying cause and the vehicle, which is why we always inspect first. Once we can see exactly what is needed we will explain it in plain English and confirm the price with you before any work starts.
