Odd brake pedal issue. Any ideas?

[silver50]

Been pleased on the whole with my new (to me) 2006 TDI estate but one issue I have is this. 

 

Sometimes when sitting with my foot on the brake, the pedal suddenly drops as though pressure suddenly drops out a bit. It doesn't completely lose pressure, it just drops down a bit, but sharply. It's ok when driving (so far!). This car has ABS (unlike my last Fabia 1) and I have noticed a slight feedback/minor kickback from the pedal when you go to touch it when driving but TBH this doesn't feel like a fault, I think maybe just a feature of the car having ABS, not 100% sure though.

 

Spoke to my local indy garage today but they were a bit puzzled. Thought maybe ABS pump.

 

Anyone experienced or reported this? Thanks

Edited 27 April, 2017 by silver50

[sepulchrave]

More likely a vacuum leak, ABS pump indeed, what a load of rubbish!

[w.t100]

If it it's a vac leak would the pedal not rise rather than drop ?? ie get harder.

[sepulchrave]

21 minutes ago, w.t100 said:

If it it's a vac leak would the pedal not rise rather than drop ?? ie get harder.

 

If the ABS isn't active then the only thing to affect pedal feel will be vacuum, and vacuum leaks are very common on these cars, also check NRV in the vac line to the servo.

[silver50]

Thanks for replies

 

Is it pretty much one main vacuum pipe I should look for coming from/going to the brake servo? NRV is presumably non return valve, how do I check it?

Edited 27 April, 2017 by silver50

[sepulchrave]

Check the NRV is doing its thing, allowing flow in one direction only.

 

One other thing to check is tyre sizes, make sure they're all exactly the same, side to side and front to back, any discrepancy confuses the ABS and it thinks a wheel is locking when you touch the brake pedal.

[w.t100]

A binding brake can also cause soggy pedal.and snatchy brake feel??

[silver50]

The tyres are all good branded make and no sign of brake binding. Feel of brakes seems absolutely fine too

 

I'll have a look in the morning at the vacuum hose. It would be good if I can see a small split or suchlike

[silver50]

Unfortunately no sign of anything amiss. Slightly tricky to see around servo but looks like a fairly solid black plastic pipe running from R/H side of engine (looks like fuel system pipes pick up pressurisation from same unit so assume it's a vacuum pump?) into the brake servo. Can't see anything inline by way of non return valve. 

 

I should get the brake fluid changed as not sure when this was done but don't necessarily see it as the culprit of this quite specific symptom