107 brake problem

[Chrono]

Help me please!

We have a weird intermittent brake problem on our 420SL.

The problem occurs only at very low speeds and seems to be more prevalent when its wet or damp, like first thing in the morning.

The symptoms are that the brake pedal falls away slightly and there's a kind of pulsing or grinding felt through the pedal and a noise like a big spring springing.

I should point out that the disks and pads are new all round and the brakes are great at all other times.

One big problem is that I can't reproduce the problem at will, so it's difficult to get it repaired because they, not unreasonably, have a drive around and everything's fine.

Has anyone got any ideas about what it might be?

 

[Brian WH]

Help me please!

We have a weird intermittent brake problem on our 420SL.

The problem occurs only at very low speeds and seems to be more prevalent when its wet or damp, like first thing in the morning.

The symptoms are that the brake pedal falls away slightly and there's a kind of pulsing or grinding felt through the pedal and a noise like a big spring springing.

I should point out that the disks and pads are new all round and the brakes are great at all other times.

One big problem is that I can't reproduce the problem at will, so it's difficult to get it repaired because they, not unreasonably, have a drive around and everything's fine.

Has anyone got any ideas about what it might be?

What you describe is the classic feel and sound of ABS working. Do you have ABS on your car? It would behave like that when the discs are wet as it would detect slippage of the brakes when applied sharply at low speed. You should be able to reproduce at say 30 mph by braking very hard and quickly.

 

[Chrono]

Yes the car does have ABS and I've been in two minds about whether this is an ABS problem.

In some respects it feels a bit like when ABS kicks in at low speeds on snow - you know that feeling when you're coming to a halt and the brakes seem to be more off than on?

But it doesn't seem to be related to any lack of adhesion on the road surface - like this afternoon when it happened in stop-start traffic on a dry road. On one application of the brakes it clonked and boinged and then it was fine for the rest of the journey.

Would this tend to suggest a specific ABS fault to investigate?

 

[Chrono]

I should also point out that it only really happens at very low speeds - like at walking pace or during parking and just as the car's being brought to a complete stop.

 

[Brian WH]

Have you inspected the discs and pads, as if they are worn/scored they could be snatching at low speeds, bringing in the ABS. Alternatively the ABS sensors could be faulty.

 

[Chrono]

I've done some searching on these boards (unfortunately you can't search on "ABS" because it's too short, so I worked through some combinations of words with ABS), anyway, it seems that the most likely cause of the problem is the ABS wheel sensors.