Brake pedal slowly going down

[Evoluzione]

Tricky one this hence me posting up.
I was sat in traffic and found the brake pedal very slowly going down when pushing and holding it. Found a rusty brake pipe with a bit of a damp patch around it so replaced it.
Bled it off by using a pressure bleeder and pumped for good measure. Still had the same issue.
Bled all four off just in case, still the same.
Wondered if it was a faulty master cylinder so replaced that.
Bled it all off again and it's still doing it!
Two things to note:
It doesn't do it with the engine off, pedal is solid. As soon as I start it up I can push the pedal down. I don't think it goes all the way.

The other odd thing is where the (new) master cylinder bolts to the servo some fluid dribbled out at the joint....
I wonder if i've picked up a faulty one and may have to resort to putting the old one back on perhaps, but i'm getting a bit fed up with rolling around on the floor outside in Winter! I'm without workshop atm.

I don't think faulty servos do this do they?

[mickyh7]

My last two cars have done this.
It's normal as far as I'm aware.
Still stops on a Sixpence and puts the antlock on!

[Evoluzione]

mickyh7 said:

My last two cars have done this.
It's normal as far as I'm aware.
Still stops on a Sixpence and puts the antlock on!

It does stop ok so maybe it's just me. It's due it's MOT though so can do without it failing.

[Gary C]

Not normal at all

The closed circuit should not allow the pedal to fall. Yes, with a the servo you can push down further but it should never reach the floor.

I had this in a Manta and the pedal would go all the way to the floor eventually and was caused by a passing seal in the master cylinder so the fluid leaked back into the reservoir so no external leak to see.

[oakdale]

What vehicle is is it?

[N7GTX]

With your engine off press the pedal several times quickly. Does the pedal firm up and rise up then, holding it pressed down does it slowly sink? If so probably air in the system or a weak master cylinder.

If it passes this test, that is no change, then:

Put your foot on the pedal. Start the engine. The pedal will go down a bit. If its a petrol it will go a short distance as the servo starts to work. If its a diesel the pedal will go much further down under hard pressing due to the vacuum pump, as much as 3/4 of the travel but then stop. If yours does this then its correct.

[oakdale]

If the master cylinder is ok and there's no fluid loss, it will be internal valve seepage in the abs unit (assuming it's a modern vehicle with it fitted), this phenomena was first noticed over fifteen years ago on many German diesels, it was mentioned in the mot testers manual and was referred to as diesel creep, testers were told to ignore it and later editions of the manual were updated to say brake pedal travel should be checked without the engine running on (petrol and diesel) vehicles with power assisted brakes.

It can happen on any vehicle with abs and isn't really a problem unless it affects the brakes in use, the situation can often be improved by braking hard a few times while driving to make the abs come into operation, this (to a certain extent) flushes and re-seats the valves in the abs unit.

[mickyh7]

Yep, my present and past cars have been German Diesels.
They both did/do the same.

[Evoluzione]

It's a 2010 Vivaro diesel, nothing special or unusual to the braking system, all pretty traditional with ABS.

N7GTX said:

With your engine off press the pedal several times quickly. Does the pedal firm up and rise up then, holding it pressed down does it slowly sink? If so probably air in the system or a weak master cylinder.

If it passes this test, that is no change, then:

Put your foot on the pedal. Start the engine. The pedal will go down a bit. If its a petrol it will go a short distance as the servo starts to work. If its a diesel the pedal will go much further down under hard pressing due to the vacuum pump, as much as 3/4 of the travel but then stop. If yours does this then its correct.

Ok thanks for the response, I think answered that in my first post, it's solid with engine off, sinks when engine is on.
Pedal doesn't go right to the floor and even when it's sunk the brakes are still operating. So yes it sounds like you describe as per the diesel example.

oakdale said:

If the master cylinder is ok and there's no fluid loss, it will be internal valve seepage in the abs unit (assuming it's a modern vehicle with it fitted), this phenomena was first noticed over fifteen years ago on many German diesels, it was mentioned in the mot testers manual and was referred to as diesel creep, testers were told to ignore it and later editions of the manual were updated to say brake pedal travel should be checked without the engine running on (petrol and diesel) vehicles with power assisted brakes.

It can happen on any vehicle with abs and isn't really a problem unless it affects the brakes in use, the situation can often be improved by braking hard a few times while driving to make the abs come into operation, this (to a certain extent) flushes and re-seats the valves in the abs unit.

Ok thanks, i'll book it in for an MOT and see how it goes. If it passes i'll ask him about it.

I'll try the O/Hs diesel car tomorrow and see if it does the same.....

[jeff666]

I replaced the master cylinder on my old Ren trafic diesel van for the same reason,(2009)

The new one did the same so tried a couple of other same make vans that my mates own , yep they all do it.

[kambites]

Our Octavia does it and always has (at least for the ten year we've had the car). I assume there's just a small amount of fluid passing the seal on the master cylinder. It doesn't appear to affect its ability to stop and it's not getting any worse so I've always just ignored it.

[Smint]

oakdale said:

If the master cylinder is ok and there's no fluid loss, it will be internal valve seepage in the abs unit (assuming it's a modern vehicle with it fitted), this phenomena was first noticed over fifteen years ago on many German diesels, it was mentioned in the mot testers manual and was referred to as diesel creep, testers were told to ignore it and later editions of the manual were updated to say brake pedal travel should be checked without the engine running on (petrol and diesel) vehicles with power assisted brakes.

It can happen on any vehicle with abs and isn't really a problem unless it affects the brakes in use, the situation can often be improved by braking hard a few times while driving to make the abs come into operation, this (to a certain extent) flushes and re-seats the valves in the abs unit.

Making the ABS work as intended makes a lot of sense.
Its also worth making a few hard stops now and again (without letting the vehicle actually come to a halt with the brakes still applied...possible material transfer), find the brakes have more bite after.