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Discussion Starter · #1 ·
Any suggestions will be welcomed .

With the engine switched off my brake pedal is good and solid , as it should be .
When the engine is switched on and you depress the brake pedal it gradually creeps down .
The brakes are good on a normal run and I am not loosing fluid .
I had the brake fluid changed completely and all corners were bleed to see if I could get a solid pedal with the engine , but it is still the same .
 

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Surely that's the way it should be? Once the engine is running you have servo assistance. Try stopping a car going downhill with the engine off - especcially a big car and it's bloomin' difficult! If the brakes are functioning properly and I assume the pedal doesn't sink to the floor when you have to stop? That would be a problem!
 

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It's a natural part of the servo building vacuum.
Iain
 

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Had the same problem. the pedal should drop down when the engine is running because the pedal pressure is increased because of the servo assist, but it should not keep going to the floor and if you release and press again and it still goes down to the floor you have a problem in the master cylinder. usually it is thethe main piston seal which is passing with the extra pressure from the servo, instead of pressurising the fluid to the brake cylinders and holding it, some fluid leaks back past the seal back into the header tank. to be on the safe side I changed mine you can not get just the rubber seals have to buy a new unit from main dealers over £100 , easy tochange but see if you can pre- bleed the unit before fitting it [look on tinternet changingandpre bleeding master cylinders]and when you bleed the corners start furthest away from master cylinderand have the ignition switched on to activate the ABS system and open the ports.
 

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Discussion Starter · #5 ·
thanks to all who replied . I tried a friends 2008 today and it did exactly the same . With the engine running and you press the brake pedal it sinks slowly .release and try it again it still sinks slowly .

Normal Braking on mine is good at the moment but I'll remember the advice on the master cylinder .
thanks again ....
 

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2008 C100 awd diesel
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Had the same sinking brake pedal and decided to replace the master brake cylinder. Did the pre-bleeding as suggested by another user. Still halfway through the job. I still have to bleed all 4 corners and check all caliper pins
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Raffo, I realise that you are a competent mechanic but for anyone else reading this topic always remember that bleeding of brake wheel calliper's / cylinders is to commence at the cylinder furthest from the main reservoir and progressively work forwards to the shortest feed line.

This particular Topic is 11 years old but is still relevant.
 

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2008 C100 awd diesel
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Agree with you on that. Only doubt I have is, since the abs unit sits under the fuse box, on the right, looking at the car from the front, should I start bleeding from the rear left (opposite abs pump) or rear right (opposite main reservoir)?
 

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You should start bleeding from the rear right as viewed from the front of the vehicle (ie opposite the ABS pump). It's not that the ABS plays any part in static bleeding, just that's the longest pipe run.
 

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2008 C100 awd diesel
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My abs pump is on the right, looking at the car from the front. Anyway, bled all four corners 3 times. Once with the engine off, with helper to push the brake pedal. Still not satisfactory. Twice with engine on, by myself with a non-return bleed valve, not satisfactory. Third time with engine on and helper to push the brake pedal. Very satisfactory. Now brakes are rock solid.

Before doing all the bleeding, I took out all the caliper pins and greased them. Some were stuck solid.

Changed the brake booster to vacuum pump pipe too as it was hard and would soon have leaked. Got all original parts from GM. No going cheap on the brake system. It's not worth the risk.

Job considered done with success.
 
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