Spongy brake pedal

[tim the pool man]

I have an issue with the brakes on my work van.

It is a Mercedes Vito, the pedal was getting long about 2 weeks ago, on checking the fluid was low so I topped it up. Since then it has got steadily worse, so I thought air had got in the system.

Thouroughly bled all 4 brakes on the weekend, but still no better.

Fluid level doesn't seem to be going down and I can't see any sign of a leak, I'm now wondering if the master cylinder might be shagged and bypassing...

Don't want to get a kit and overhaul it to find it no better, any advice?

[sherman]

I would say its either the master or the slave cylinder. It could also be that the pads are shagged as well.

[dan19evans]

When was the fluid last changed?
Does it get worse as you drive?

[deviant]

Pump the brakes up hard and then stand on the pedal. If it slowly sinks down then fluid is getting out or bypassing something.

[tim the pool man]

Finally fixed this...
Couldn't source a kit for the master cyl from Perth Brake Parts, Auto1 or Mercedes Parts, new M/C almost $500...
So was going to take the MC into PBP to match up seals and maybe hone if reqd.

Jacked up to take LHF wheel off to drain fluid... hmm, wheel seems to be wobbling a bit (hadn't noticed anything while driving)

Bloody big nut which holds the drive shaft in place had loosened, causing the disc to wobble and knock the pads back, tightened it (and loctited it) and pedal now perfect...

Can't believe that 1 the wobble wasn't at all evident while driving
and 2 we didn't notice it when we first bled the brakes...
must be getting old

[deviant]

There should be a split pin holding the nut in place? On all cars I have looked at the nut is casselated and there is a hole in the drive shaft for the split pin to pass through.

[tim the pool man]

No this one is a nyloc, I will get a couple of new ones soon from Merc

[thinfourth2]

Your bearing is probably knackered as adjustable bearngs are a rare thing on modern cars

[Jimmyarm]

Slight necro but this thread came up on a google search.

Had a similar issue with a customers 2008 Vito, failed the MOT as the pedal had excessive travel.

This has the Brembo brake setup (some have Bosch) and the bottom piston on both calipers was siezed solid, the top one working fine so it passes the roller test. I don't know enough about hydraulics to explain why but leaving the pedal untouched for a minute or so meant the top piston retracted back 'excessively' causing the first brake application to take up that slack and cause the excessive travel.

Fairly easy to find by clamping the hoses off