Mercedes Sprinter Brake Pedal Goes Down To The Floor!

[MantaRay]

Here's a puzzler for you, well, me anyway. I've just fitted a new brake master cylinder to my 2003 Mercedes Sprinter van, bled the system through, and achieved a nice solid pedal. However, started up the engine, gained some servo assistance and the solid pedal has disappeared, although there is initially "normal" braking. Keeping light pressure on the pedal it gradually sinks to the floor. Without servo assistance the pedal is solid, even when pushing on the pedal as hard as I can. What's going on?

[tonyav]

Have you checked the pipes on the servo unit for leaks ?, can you hear any hissing noises when you put your foot on the brake when the engine is running.
Definitely sounds like a prob with the servo unit.

[Hymie]

All brake pedals with servo assist will require significant force to depress them with no vacuum from the manifold.

Many moons ago, when I would spend my weekends crawling under my old Ford – I suffered a similar problem, having bled the brakes. Despite having no air bubbles in the fluid at the bleed nipple – air was still trapped within the system, causing a spongy brake pedal.

I would advise that you do as I did more than 30 years ago – buy some more brake fluid and bleed them again. Make sure that you are bleeding the brakes in the sequence set out in the service manual – if so required by the vehicle.

[MantaRay]

Thanks for your comments tonyav & Hymie.

I have checked the servo for leaks, no signs of any problems there. Servo works when it should and holds the vacuum okay after having switched off the engine. No strange noises either. Will bleed all four calipers again and see what happens, maybe there is air trapped in the system somewhere?

[Wordist]

My Renault Master does the same thing. It works ok on the road and passed the MOT test with no comment, so I assumed it was normal.

[slocjo]

hello mate, I worked for a Mercedes dealer for 17yrs...which was 11yrs ago..if I remember we had to pressure bleed the system due to the large abs valve...if no joy, give me a shout back & I will contact a mate who still works there...