C4 - Hard braking pedal on starting the car

[lancia58]

Hi

A 2009 C4 1.6 SX automatic. Quiet often when starting the car the braking pedal feels hard. The pedal should be pressed hard to stop the car. After about 20 seconds it feels noraml. Is this a knowen problem ?

Thanks

[myglaren]

Have you tried braking hard and repeatedly?
Does the pedal go hard after a couple of presses?
If so it is likely that there is an air leak in the brake servo line or a fault with the servo itself.

[lancia58]

Hi

Yes I tried , but it didn't help. The pedal stays hard for about 20 seconds and then it is normal. If there is an air leak how comes that after 20 seconds all is normal ? A mechanic that works for Citroen told be that the cause may be a carbon build on the engine valves but I cannot see the logic behind

Thanks.

[myglaren]

Hi

Yes I tried , but it didn't help. The pedal stays hard for about 20 seconds and then it is normal. If there is an air leak how comes that after 20 seconds all is normal ?

Because it interferes with the time taken for the servo to build sufficient depression ('vacuum')to operate.

A mechanic that works for Citroen told be that the cause may be a carbon build on the engine valves but I cannot see the logic behind

Thanks.

Agree with that. I can't see that having a significant effect on manifold depression without noticeable effect on running.
Also depends on if your car uses manifold depression or has an additional vacuum pump.
If the latter then the pump may not be operating properly.

I can't say I have ever experienced your problem personally other than the pedal going hard when the engine is stopped. Can't say I apply the brake before setting off after just starting the engine. Only use the handbrake then. Different with an automatic perhaps.

[xantia_v6]

If the vaccum comes from the manifold (rather than a pump) on this model, then there will be a non-return valve in the line, which could be partially blocked.

[lancia58]

So how comes that after 20 seconds all is normal ?

Thanks

[RichardW]

It could just be that it takes this long to build up the vacuum in the servo - you will probably find that if you pump the pedal repeatedly with the engine running the servo assistance will temporarily disappear. If you hold the pedal down and the start the engine, does the pedal sink when the vac comes on? It should do this immediately, but I suspect yours will take the few seconds. XantiaV6s suggestion is probably a good place to start - I would imagine that a 1.6 petrol will have manifold vacuum not a vac pump. It might also have a vac reservoir - these are prone to cracking which could give the symptoms you are describing in that it takes time to build the vacuum.

[Lighty]

I have had a situation where the vac pump has a metal filter that get blocked inside the pump , obviously not if its a petrol model.

[lancia58]

Yes it is a manifold vacuum