Vito brake pedal spongey, but only with engine running?!
Discussion
My W639 Vito has a really spongey brake pedal. I have bled the brakes, twice now. Once with the engine off then again with it running to make sure the ABS pump is primed. If I pump the pedal with the engine off, it stays rock solid. If I do it with the engine ON, it slowly sinks right down. I'm a bit puzzled and have an MOT coming up on Saturday morning so need to figure this out ASAP.
Am I right in thinking that it can't be the master cylinder since the pedal won't sink with the engine off?? I also tried greasing the sliders on the front calipers and I don't believe either front caliper is seized, having had a look at them last night.
Any ideas anyone? I do have a brand new MC I can throw on but I'd rather return it for a refund if it's not the problem. If it is, then I'll swap it tonight.
Am I right in thinking that it can't be the master cylinder since the pedal won't sink with the engine off?? I also tried greasing the sliders on the front calipers and I don't believe either front caliper is seized, having had a look at them last night.
Any ideas anyone? I do have a brand new MC I can throw on but I'd rather return it for a refund if it's not the problem. If it is, then I'll swap it tonight.
Probably master cylinder on the way out. You're generating more line pressure with the engine running and the servo working for a given pedal input and it's likely blowing past the seals in the M/C.
If you really stand on the pedal with the servo depleted you will probably find that it still creeps.
The only alternative is that something odd is going on with the servo
If you really stand on the pedal with the servo depleted you will probably find that it still creeps.
The only alternative is that something odd is going on with the servo
The Wookie said:
Probably master cylinder on the way out. You're generating more line pressure with the engine running and the servo working for a given pedal input and it's likely blowing past the seals in the M/C.
If you really stand on the pedal with the servo depleted you will probably find that it still creeps.
The only alternative is that something odd is going on with the servo
I wondered about the "generating more pressure" thing. Sod it, I'll bite the bullet and change it. If you really stand on the pedal with the servo depleted you will probably find that it still creeps.
The only alternative is that something odd is going on with the servo
I had the same problem to this on a Volvo S40 T5, the T5 bit is important, and concluded the same thing, the master cylinder was dying. So, I fitted a new master cylinder, which was a real slag of a job. No change. But, as with the OP it was only a problem with the engine running.
It was my wife’s car, I’ve got a Mondeo with the same engine, and I noticed hers had a vacuum pump between the engine and servo, which is odd for a petrol engine, but mine didn’t. So, I unplugged the vacuum pump on the Volvo and the problem went away, and in turn improved the brake pedal feel. Whether the issue was the vacuum pump itself or if there was any cables or such like in the line I am not sure.
Have look to see if you have got a vacuum pump, and what disconnecting it does.
It was my wife’s car, I’ve got a Mondeo with the same engine, and I noticed hers had a vacuum pump between the engine and servo, which is odd for a petrol engine, but mine didn’t. So, I unplugged the vacuum pump on the Volvo and the problem went away, and in turn improved the brake pedal feel. Whether the issue was the vacuum pump itself or if there was any cables or such like in the line I am not sure.
Have look to see if you have got a vacuum pump, and what disconnecting it does.
TroubledSoul said:
Hmm... I don't really want to be disconnecting anything though. I'd rather fix it properly. I guess as a test I can try that but it's not something I'd want to be using as a bodge.
Yes, on a diesel it would be disconnected for testing only. I felt comfortable disconnecting on the Volvo because the power train of the Volvo and Mondeo are basically the same, the Mondeo is heavier, and Ford felt that could cope without the additional pump. The Volvo went on for a couple of years with us afterwards, passing MOT’s with flying colours.It is definitely something to look for though, and it wouldn’t even require driving it if your concerned.
Megaflow said:
and I noticed hers had a vacuum pump between the engine and servo, which is odd for a petrol engine, but mine didn’t.
Actually very common now. I'm presuming to do with all this stop start nonsense. A lot of cars run the pump....and many the exact same pump.Means you can always have vacuum for brake assistance, even when the engine may not be running during normal operation.
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