Sudden brake loss

[Cableguy]

Hi all, just got back from another fab weekend away. Whilst on the motorway, I went to brake and the pedal went straight to the floor, brakes still just about worked, but scared me silly. Happened every time I braked and coincided with a big increase in idle speed. In a traffic queue the idle speed shot up to 2000. We decided to pull in to the services and couldn't find anything obvious. Started it up again and both problems disappeared.
Brakes fine all the way home, any ideas of what might be happening?
Going to Wales soon and need to be sure I'm safe.

[project25]

Hi, cant help you cos I havent got my T25 going yet but there was quite a long heated discussion on here somewhere about brake loss, you might find it in the search section. Dont happen to know anything about autos Im waiting for some advise.

[Cableguy]

I did look, but couldn't see it. I do remember the thread though. Will have another look. Thanks

[California Dreamin]

Posibilities:

Brakes are used very little whilst on the motorway and remain pretty cool. so failure under these circumstances is rare.....

Theory 1
Is it possible you have a partially seized caliper or your handbrake was binding perhaps? leading to vapour lock (the brake fluid boiling and the gassious bubbles released compress under braking)

Theory 2
Low brake fluid in the resevoir exposing the ports but you should see a light come onand I guess you have already checked the level.

Theory 3
Master cylinder catastrophic failure (poss seal damage caused by a recent brake fluid change, where full brake travel under pressure has led to seal damage)

Theory 4
Brake pipe balooning, doesn't fix it's self though.

Brake pipe burst but again...doesn't fix itself.

The revs increasing is undoubtedly down to air being sucked down the brake servo pipe into the inlet manifold.
Understand that vacuum servo failure results in a hard/solid brake pedal and not the brake pedal going to the floor.
It is posible for the main seal in the master cylinder to leak into the servo itself.... shows as fluid loss but with no visable external leak.

Hope this helps.
Martin

[croc]

Think California has covered the most likely causes.
Logically, the vacuum servo is just that, a servo to apply additional brake power. Failure of this only results in really heavy brake pressure being required, on the pedal, but does not make pedal go to floor with little force. Same as applying brakes with engine stopped.
I realize this doesn't solve the issue but might eliminate one aspect. Sorry I don't recall the failure thread either.
Good luck and please do report the final solution here so we learn too.
jon

[Cableguy]

Found the thread, it refers to failure following bleeding brakes.
Haven't touched anything except a new handbrake cable.
I agree with the above list of possibilities, but can't be any of those if the brakes now work fine.
Must be something to do with the servo as it corresponded with high idle speed.
Don't really understand what's going on. Will check the vacuum hoses and see what's what.
Thanks for your replys guys:-)

[croc]

more ... !
on further thought, you may have two separate issues here.
California's theory 3 - master cylinder failure, maybe due to deterioration of seals and other piston parts or ports blocked with gunk.
this could be intermitent as whatever it is blocking mc ports may clear and then block again?
internal cup or seal shredded and the pieces blocking ports?
It may be coincidental with the servo or servo hose failure giving higher revs due to air leak.
but you've probably covered these ...

[Cableguy]

Ok, checked van over.
No ballooning hoses
No signs of leakage
no loss of fluid
can't see how it can be master cylinder as brakes worked fine for last 70 miles of the journey.
Increased rpm on idle sounds like a servo issue, but all posts suggest pedal goes hard if servo playing up.

[BOXY]

Haven't touched anything except a new handbrake cable.

Could fitting the new cable too tightly have led to the brakes being partial held on. This then leads to the brakes getting gradually hotter until the brake fluid boils as per:-

Theory 1
Is it possible you have a partially seized caliper or your handbrake was binding perhaps? leading to vapour lock (the brake fluid boiling and the gassious bubbles released compress under braking)

[California Dreamin]

F.Y.I......brake fluid has an unfortunate 'property'... it is hygroscopic...which means it attracts and absorbs moisture (water) from the atmosphere.
Now then....DOT4 brake fluid boils at around 230 degrees C and water, just 100 C, so water in brake fluid is not a good thing.
As far as 'testing' goes, brake fluid has 'two' boiling points, dry and 'wet'. Wet refers to 3.7% water contamination by volume, which effects the overall boiling point by an incredible 75 degrees C dropping it from the normal 'dry' boiling point of 230 degrees C to just 155C.
It doesn't take a rocket scientist to work out the implications this might have on a hot braking system where vapour lock is a real issue on these small and over worked braking systems.
Therefore it is vitally important replacement schedules are properly adhered to...... brake fluid needs changing every two years regardless of mileage.

Martin

[kit]

Is it a diesel?

[Cableguy]

No it's petrol

[kit]

Ok it is not the vacuum pump then.

[Cableguy]

Just to update on this:
Turned out to be seized calipers. Only outside pads on both front brakes were making contact. Brakes were 'serviced' by a garage before this, so I had no reason to suspect them. Just shows you really.
Obviously this meant loads of pedal effort to stop and the heat was probably boiling the fluid. Have done the master cylinder and fitted new vented passat brakes. It stops now

Thanks for all the suggestions, it's always the obvious ones