W211 (09) Brake Problem

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E55BOF

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I've just bought a very late W211 estate (no SBC), and it has a brake problem. The selling dealer will have to pay for the repair and claim it back from the warranty company, but I'd like to have some idea of what's likely to be involved before it goes to Terry Gates for the work.

The brakes work perfectly, and there are no dash warning lights on at all when the car is running. However, brake pedal travel is longer than I'm used to, and if I keep heavy pressure on the pedal after the car has stopped it will sink slowly and steadily to very near the floor with no increase in resistance. (I'm a big beggar with strong leg muscles...).

If I switch off, then pump the pedal until the servo effect is exhausted, then pump it to take up the excess travel, it goes hard, and then heavy pressure will not budge it any further down.

The second, non-servo, brake actuation certainly suggests air in the brake system, which bleeding should fix, but I'm rather puzzled by what happens when the brakes have servo assistance. Can anybody shed any light on this peculiar symptom?
 
And there's more....

I always (yes, I'm a boring old fart..) draw up a list of every service and MoT I have evidence of for any car I buy, and the list may also indicate the cause of the spongy brakes. To be fair, they work perfectly, and a not particularly aware owner might either not notice the altered pedal feel, or believe that it was down to new pads and discs needing to wear in.

At the last MoT, six months and 10k miles ago, the car failed initially, with an advisory on front disc & pad wear. That advisory is not on the retest pass certificate, and the front discs are very little worn. I suspect that the front brake bleed valves were opened to release fluid so the pistons would retract into the calipers, somehow air got in, the brakes were not bled properly (vacuum rather than pressure bled, perhaps) and the slightly spongy, long-travel pedal was ‘overlooked’ on retest…
 
Solved, but not quite as I expected. The rear discs were well-worn (which I knew), and on inspection, Terry Gates found that the rear pads were unevenly worn as well. New brakes on the rear, and all is well.

HOWEVER... The brake pedal travel is now normal, but it still goes slowly down if you keep the pressure on after stopping! It looks as though it's just a characteristic of the system.
 

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