Brake pad glazing and disc surface damage

This is a little-known problem which is common on little-used cars, and which reduces maximum braking effort to a substantial degree.

Most good drivers – especially of classic cars – use their brakes sparingly and with due consideration for the mechanics of their cars and potential poor grip. However, under this sort of regime, disc pads (and drum shoes) can become ‘glazed’, which is to say shiny, with reduced friction. When brakes are applied suddenly and hard it is usually because there is an emergency – not a good time to have reduced braking effect!

The solution is stamp on the brakes occasionally – and at least every 100 miles of sedate driving – three or four times, with due concern for safety if the car should swerve or not brake as quickly as expected. This heats up the pads, and they shed their ‘glazing’. It also exercises the brake system, freeing up potentially sticking calliper pistons, and showing up any potential faults – especially in the ABS system – under controlled conditions. You won’t do any harm to the system by being violent occasionally, and it will definitely benefit braking.

A related problem is where the brake discs become uneven in friction around the braking surface. This gives rise to a pulsing effect, not dissimilar to wheel wobble. Let’s call it brake wobble. The most common reason for this is where after a long, hot braking episode, the brakes are left applied (common with automatic transmission cars) for some minutes, at traffic lights for example. The very hot disc overheats the pads which give off corrosive chemicals which change the surface of the disc – but in only the area occupied by the pads at that time. When you drive off, the affected area has different friction from the rest, so braking again gives rise to this varying-friction brake wobble.

The solution – apart from new discs, or having them skimmed – it to brake differently. The technique is to brake as you need, but after a strong braking episode, release the footbrake and hold the car on the handbrake or in Park. It is best to use Park (on an automatic) since even the handbrake pads can cause the same effect on hot discs, but the rear discs don’t get as hot, and the handbrake pads are smaller.