What is causing my car’s brakes to grab?

2016 Seat Leon
Alex Robbins suspects the issue lies with the Seat's brake servo

Dear Alex,

I have a 2016 Seat Leon with grabby brakes. This started after its last service and seems to be getting worse. I have tried (safe) emergency braking in case the pads are glazed but no change. The garage says different pads won’t help. Any ideas?

– CM

Dear CM,

Grabby brakes are often the result of a sticking caliper, but that tends to mean the brakes will grab on only one side, causing the car to feel as though it’s skewing right or left, or affecting the steering.

So I wonder whether the issue lies with the brake servo. This device amplifies the amount with which you are pressing the brake pedal, so that you do not have to push it quite so hard.

It works on the principle of pressure change. Inside is a large, flat, circular piston, sitting between two chambers. One of these chambers, on the side of the brake master cylinder, contains a constant vacuum when the engine is turned on, created by a hose linked to the inlet manifold; the action of the engine, which is trying to draw in air, naturally sucks air out of the chamber.

The other chamber, on the opposite side of the piston, also contains a vacuum when the brake pedal isn’t being pressed, so the pressure is equalised between the two chambers. But the action of pressing the brake pedal opens a poppet valve, allowing this second chamber to fill with air at atmospheric pressure.

This pressure then forces the piston to move towards the brake master cylinder. On the other side of the piston, another pushrod then acts on the master cylinder, thus the force with which you are activating the brakes is increased.

Of course, the amount of air allowed in has to be rationed. The poppet valve is set up to do just that – otherwise, you would get maximum braking the second you touched the pedal, which would not be ideal.

So I wonder whether the valve may be allowing too much pressure into the servo too soon – causing the brakes to grab. If the valve is failing slowly, this would fit with your description of the problem getting worse gradually.

Unfortunately, I suspect the only fix is a complete new servo. In fact, it will probably be cheaper to do that than to pay someone to take it apart and replace only the valve, which will be an involved and painstaking job.

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