[Black Sapphire]

Hi all

When I return to my car after it been parked up I put my foot on the brake pedal before I start the car (as you need to in the auto) and sometimes the pedal is rock hard?

As if someone has been pumping the pedal without the engine running.
When the engine starts the pedal sinks (as it should) returning to the normal position.

This happens randomly and I can see no pattern or reason for it.

Anyone else had this? Or knowguess why this is happening?

[dxb335d]

Yeh i think i have mate, not really thought bout it tho! or even take much notice!

[ShereKhan]

Maybe the brake servo's may be applying some pressure on the fluid keeping the pedal rock hard. When you start the engine the servo's will be running keeping the pedal soft.

I've rolled in traffic (with engine off) and nearly panicked when the car nearly failed to stop

So never try rolling down a hill with engine off

[vivekk]

Originally Posted by Black Sapphire

Hi all

When I return to my car after it been parked up I put my foot on the brake pedal before I start the car (as you need to in the auto) and sometimes the pedal is rock hard?

As if someone has been pumping the pedal without the engine running.
When the engine starts the pedal sinks (as it should) returning to the normal position.

This happens randomly and I can see no pattern or reason for it.

Anyone else had this? Or knowguess why this is happening?

This is a known issue & has been discussed previously.

Have a look here.

Viv

[beemerbird]

I remember that thread Viv, but the guy never came back with the answer.

I've taken mine to the dealers for exactly the same thing and they couldn't find anything wrong with it but it still does it occasionally, although the brakes operate as they should whilst in use etc.

I've noticed though that if you pull up somewhere and happen to press the brake pedal more than a couple of times whilst parking for instance (even gently) then switch off the engine, when you return to the car and foot on brake pedal to start again, the pedal is 'hard'.

Buggered if I know.

[quattrogmbh]

Diesels don't build up vacuum in the same way as a petrol. They have a vacuum resevoir instead. Its a black canister like thing left hand side under the engine cover. Over a period of time, when the engine is switched off, the resevoir bleeds to atmosphere, leaving no residual vacuum in the servo as you'd have with a petrol. Its designed to do this and perfectly normal.

On the other thread it seems as though the issue is with the tolerances of the starter inhibit switch, rather than the braking system.

[beemerbird]

Thanks for the info.

I've looked at that thread again and that guy had trouble starting his car - hence your ref. to the starter inhibit switch. Mine's fine in this department.

[red-max]

I've noticed this occasionally too... not sure of the reason though.

I've also noticed the brake pedal is a lot more sensitive when cruise control is on ... nearly stood car on it's nose a couple of times.

[quattrogmbh]

This one is a side effect of the "Brake Standby" feature of the latest Bosch ABS. The dynamic cruise control uses this feature to move the pads closer to the brake disc so that less pressure is required when braking is activated by the cruise control brake function.
So when you hit the brakes and cruise is on, the biting point is different to you have normally without cruise.. Does take a bit of getting used to.

Originally Posted by red-max

I've noticed this occasionally too... not sure of the reason though.

I've also noticed the brake pedal is a lot more sensitive when cruise control is on ... nearly stood car on it's nose a couple of times.

[NFS]

Originally Posted by quattrogmbh

This one is a side effect of the "Brake Standby" feature of the latest Bosch ABS. The dynamic cruise control uses this feature to move the pads closer to the brake disc so that less pressure is required when braking is activated by the cruise control brake function.
So when you hit the brakes and cruise is on, the biting point is different to you have normally without cruise.. Does take a bit of getting used to.

This is the same brake priming effect that you get if you rapidly lift your foot of the accelerator before braking.

My mate drove my car recently and accidently hammered on the brakes because of this.

[Black Sapphire]

Originally Posted by quattrogmbh

Diesels don't build up vacuum in the same way as a petrol. They have a vacuum resevoir instead. Its a black canister like thing left hand side under the engine cover. Over a period of time, when the engine is switched off, the resevoir bleeds to atmosphere, leaving no residual vacuum in the servo as you'd have with a petrol. Its designed to do this and perfectly normal.

On the other thread it seems as though the issue is with the tolerances of the starter inhibit switch, rather than the braking system.

Hi quattro

You say this is with diesels, do you mean all diesels or just BMW's?
I ask this as I've never had this with other diesels I've driven.

Also, do you why the diesel would run a different brake servo system to the petrol?

Thanks all for your responses.

[c-hri-s]

I've had this a couple of times - seems especially when I rush into the car, shove the key in, put my foot on the brake the starter won't operate. Normally I take my foot off, put it back on the pedal and then it starts just fine.

[quattrogmbh]

The method of creating vacuum is typical of all Diesels. A pump is used, rather than being obtained from the manifold. The method in which the vacuum resevoir is bled may well differ between manufacturers. I know from experience that BMW and VAG work in the same way. It really depends on how long the vacuum resevoir is kept primed. As soon as it is vented to atmosphere, you'll have the hard pedal feel you describe.

Try the same on a petrol car by depressing the brake several times with the engine off. The same will happen.

I don't believe the brake servo is any different between the 335D and 335i, its just the way of providing vacuum to the servo which differs.

The 335i is a bit of a special case here, in that it does have a vacuum pump but is only used as part of the turbo control mechanism.

Originally Posted by Black Sapphire

Hi quattro

You say this is with diesels, do you mean all diesels or just BMW's?
I ask this as I've never had this with other diesels I've driven.

Also, do you why the diesel would run a different brake servo system to the petrol?

Thanks all for your responses.

[Black Sapphire]

Originally Posted by quattrogmbh

The method of creating vacuum is typical of all Diesels. A pump is used, rather than being obtained from the manifold. The method in which the vacuum resevoir is bled may well differ between manufacturers. I know from experience that BMW and VAG work in the same way. It really depends on how long the vacuum resevoir is kept primed. As soon as it is vented to atmosphere, you'll have the hard pedal feel you describe.

Try the same on a petrol car by depressing the brake several times with the engine off. The same will happen.

I don't believe the brake servo is any different between the 335D and 335i, its just the way of providing vacuum to the servo which differs.

The 335i is a bit of a special case here, in that it does have a vacuum pump but is only used as part of the turbo control mechanism.

Thanks quattro,

I understood whats happening (use the pedal pumping exercise to test the servo when purchasing 2nd hand cars ) just didn't understand why it would differ on the diesel.

One more question for you (oh knowledgeable one!)
why can't it keep it primed until you next start the car?
Also why does the timespan differ? I can return after 4 hours or 20 hours and sometimes its hard sometimes its not!

ok thats two questions!!

[quattrogmbh]

You're welcome...

The timing is something that I can't answer.. I would imagine a sensor somewhere for something tells the resevoir to vent. Its not considered good practice to keep something under pressure or vacuum unless its really necessary. Since the resevoir is primed as soon as the engine starts, I guess its considered acceptable as you won't be needing brakes before then ;-)
Same happens with the fuel lines except they are primed as you open the drivers door. On the diesel, there is no reason that the vacuum couldnt be replenished at the same time since its an electric pump. Something which isnt possible with the petrol, and I suspect the root of the reason why its not done in advance - keeps control systems common between variants.

Originally Posted by Black Sapphire

Thanks quattro,

I understood whats happening (use the pedal pumping exercise to test the servo when purchasing 2nd hand cars ) just didn't understand why it would differ on the diesel.

One more question for you (oh knowledgeable one!)
why can't it keep it primed until you next start the car?
Also why does the timespan differ? I can return after 4 hours or 20 hours and sometimes its hard sometimes its not!

ok thats two questions!!

[Black Sapphire]

Originally Posted by quattrogmbh

You're welcome...

The timing is something that I can't answer.. I would imagine a sensor somewhere for something tells the resevoir to vent. Its not considered good practice to keep something under pressure or vacuum unless its really necessary. Since the resevoir is primed as soon as the engine starts, I guess its considered acceptable as you won't be needing brakes before then ;-)
Same happens with the fuel lines except they are primed as you open the drivers door. On the diesel, there is no reason that the vacuum couldnt be replenished at the same time since its an electric pump. Something which isnt possible with the petrol, and I suspect the root of the reason why its not done in advance - keeps control systems common between variants.

Thanks again quattro!!

[Ant397]

Originally Posted by quattrogmbh

You're welcome...

The timing is something that I can't answer.. I would imagine a sensor somewhere for something tells the resevoir to vent. Its not considered good practice to keep something under pressure or vacuum unless its really necessary. Since the resevoir is primed as soon as the engine starts, I guess its considered acceptable as you won't be needing brakes before then ;-)
Same happens with the fuel lines except they are primed as you open the drivers door. On the diesel, there is no reason that the vacuum couldnt be replenished at the same time since its an electric pump. Something which isnt possible with the petrol, and I suspect the root of the reason why its not done in advance - keeps control systems common between variants.

Good points all round, but I think the problem comes down to mainly auto's rather than manuals in the way that you have to keep the brake pedal pressed which pressurises the brake lines (the rubber parts expand) and then still have the brake pressed when the engine is switched off and the brake pedal is used to push yourself out of the car once you have opened the door, so there still be pressure in the brakeing system that isnt released as there is no vacume control. The same when you press the pedal a couple of times without the engine running - the pedal goes hard and stays hard.

You are right that diesels need an alternate way of generating a vacuum and stroring it, but this is done mechanicaly not electricaly.

If you open the bonnet on a 330d for example, look between the radiator cowling and the engine cover, on the front of the engine just infront of where the exhaust cam shaft would be and you will see the goldish coloured circular mechanical vacuum pump that opperates the vacuum system.


ant.

[Black Sapphire]

[QUOTE=and then still have the brake pressed when the engine is switched off and the brake pedal is used to push yourself out of the car once you have opened the door,
ant.[/QUOTE]


See your point but not sure if I use the brake pedal to push myself out of the car!!

[quattrogmbh]

Originally Posted by Ant397

You are right that diesels need an alternate way of generating a vacuum and stroring it, but this is done mechanicaly not electricaly.

ant.

Good Point Ant, Not sure what I was thinking there :-)
Here is the pump referred : http://www.realoem.com/bmw/showparts...72&hg=11&fg=45

[Ant397]

Originally Posted by quattrogmbh

Good Point Ant, Not sure what I was thinking there :-)
Here is the pump referred : http://www.realoem.com/bmw/showparts...72&hg=11&fg=45

Thats the one! Gets mad hot too!