W639 Vito brake pedal travel

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Druid16

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May 29, 2015
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Car
Vito
2005 Vito 111CDi brake pedal sinks to the floor. I've bled the complete brake system and there's a good firm pedal with the engine off, but when the engine is running the pedal sinks to the floor. Pumping the brakes brings the pedal up, but it sinks straight down. The brakes bite when the pedal has moved through half its travel, and there are no fluid leaks.

It's my wife's daily transport and she says it's always been like that but that doesn't mean it's right. I'm just looking for some advice before I fork out for a new master cylinder and find I still have the same issue.
 
It should not be doing that.
With the pedal depressed, on starting the engine, the pedal should firm up and lift. I am not sure that the master cylinder is your problem, though. Sounds more like the servo or the vacuum for it at a guess.
My reasoning is this; the master cylinder either works or doesn't whilst the servo system relies on vacuum which works only with the engine running….it's why when towing with the engine off, the brakes have very little power, because there is no servo assistance. Conversely, if you have firm pressure with the engine off but spongy with engine running, the servo or it's accompany vacuum system is at fault, I believe.
 

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