Diesel Pedal Creep?

[Parkgate]

Ok, I've finished the brake lines, changed the brake fluid, bled the system twice (pressure bled) and everything is rock hard! Turn the engine on and the brakes seem to work great. I took it up and down the road a few times, slamming the anchors on, and everything is fine including the ABS, but if I hold the pedal down it slowly sinks!
I can't remember it doing that before (sinking) BUT i must admit I can't recall ever a) hitting the brakes hard enough to trigger the ABS or b) holding the pedal under pressure like that before.. I just normally put it in P if waiting instead of pressing the brake pedal down hard ..is it me or do I have a brake system problem?
I can't see any weeps, the lines have been bled twice and without the engine running its solid. I need to get the car MOT'ed tomorrow, so is this pedal creep normal in a merc diesel or will it fail? I've just been reading about 'diesel pedal creep' but seeing how I've done so much work on the brakes and lines I could do with some input in case I've missed something!!

 

[Chris & Sue]

Both mine do it, is it solid without the engine running, if so its just the servo working.

 

[Taffy7hfa]

I've not heard of diesel pedal creep, I would have assumed that there was a problem in the system somewhere

 

[Parkgate]

Both mine do it, is it solid without the engine running, if so its just the servo working.

Yes, its solid when the engine isn't running. But as I've done all the brake line work myself (lying on my back over the last week) maybe I'm being (rightly so imho) slightly over paranoid?

 

[rf065]

Mine does that too, perfectly normal as long as it's solid with the engine off.

Russ

 

[Parkgate]

I've not heard of diesel pedal creep, I would have assumed that there was a problem in the system somewhere

Thats what I first thought, but the only thing I can think of is air in the ABS somewhere but I have no I idea how to purge it, if there is?

 

[rf065]

Thats what I first thought, but the only thing I can think of is air in the ABS somewhere but I have no I idea how to purge it, if there is?

Read this from the gov MOT advice page............

Vehicles with petrol engines uses the inlet manifold to generate the vacuum for servo assistance.
Larger vehicles such as vans and 4x4s with diesel engines usually use a pump to generate the vacuum, Unlike the inlet manifold of a petrol engine, there is no vacuum relief with a pump. If excessive pedal pressure is applied when the vehicle is stationary (and the engine is running) as required for the MOT test the hydraulic pressure required to stop the vehicle will be grossly exceeded and fluid will be forced past seals that are between circuits.

The rare occurrence is known as diesel creep; and it is often incorrectly diagnosed as being caused by a faulty master cylinder.

The solution is to stop applying the excessive pressure.

If you can get the brake pedal to creep with the engine switched off and servo exhausted,
or actually under braking there is a serious problem that requires urgent attention!

A road test maybe an option to satisfy yourself, but if in doubt give the benefit of the doubt to the vehicle presenter.

Russ

 

[Parkgate]

Read this from the gov MOT advice page............

Vehicles with petrol engines uses the inlet manifold to generate the vacuum for servo assistance.
Larger vehicles such as vans and 4x4s with diesel engines usually use a pump to generate the vacuum, Unlike the inlet manifold of a petrol engine, there is no vacuum relief with a pump. If excessive pedal pressure is applied when the vehicle is stationary (and the engine is running) as required for the MOT test the hydraulic pressure required to stop the vehicle will be grossly exceeded and fluid will be forced past seals that are between circuits.

The rare occurrence is known as diesel creep; and it is often incorrectly diagnosed as being caused by a faulty master cylinder.

The solution is to stop applying the excessive pressure.

If you can get the brake pedal to creep with the engine switched off and servo exhausted,
or actually under braking there is a serious problem that requires urgent attention!

A road test maybe an option to satisfy yourself, but if in doubt give the benefit of the doubt to the vehicle presenter.

Russ

Can you do me a favour and press down hard on the brake pedal with the engine running and see if it sinks as your sig car CLK220CDI is basically the same car less a cylinder! Cheers! OOps my bad it IS the same car, the phone just went and I wasn't wearing my glasses when I read you post!

 

[rf065]

Can you do me a favour and press down hard on the brake pedal with the engine running and see if it sinks as your sig car CLK220CDI is basically the same car less a cylinder! Cheers!

Read post number 5!

Russ

 

[Parkgate]

Read post number 5!

Russ

Lol, I've got my mate on the phone telling me about brakes AND I'm trying to type on here at the same time AND I NEED to put my reading glasses on!

 

[rf065]

Lol, I've got my mate on the phone telling me about brakes AND I'm trying to type on here at the same time AND I NEED to put my reading glasses on!

Read post 2 as well while your glasses are on!

Russ

 

[Parkgate]

Glasses on, phone call ended...(I'm hopeless at multi-tasking!) yes Russ I see a pattern emerging with diesel brakes and your foot down hard on the pedal!

Rob

 

[om613]

I had that. Passed MOT fine, which after all measures the brake effort.
Wasn't happy with the pedal feel, so bled them again. All was then much improved.

It's bedding in of the wearing elements / bleeding out the last remnants of air, I think.

 

[Taffy7hfa]

I guess its something to do with the vacuum being generated by an engine driven pump rather than the inlet manifold as with a petrol engine.

 

[Parkgate]

Well after bleeding the brakes again this morning I pulled a little more air out of all four corners. Did I mention how cold it was yesterday and today? Anyway the pedal still sunk slowly when stationary with a firm foot on it, so I put it down to diesel pedal creep, took it this afternoon for its second re-test and it passed its MOT with flying colours!
My guess is air does get trapped in the system and just takes a little time to work its way to the calipers where it can be bled out as OM613 said. And diesel pedal creep compounds the matter making you scratch you head in confusion!
Anyway its passed the MOT and now I'm ****** sick of car brakes and brake lines, feet up time!!