Brake pedal Intermittently goes to the floor.

[Kpb]

I I will cut a long story short.

My 2006 9n3 gti recently had new discs and pads fitted to the rear. All done propperly did it along with me mechanic freind. And then a garage blead the brakes for me.

That was a few weeks ago. In the last 4 days I've had the pedal sink to the floor with little to no braking force. When you bring the peddle back up and then re-apply it it is fine as if there was nothing wrong.

Things I have checked.
Vacume lines to the booster from the manifold.(no splits or cracks)
Brake lines and callipers (no leaks)
Fluid level (has not changed a mm)
Fault codes (non)
And checked the master cylinder for leaks. (nothing)
Pumped the brakes when the car is turned off and the peddle goes fairly hard but has a very slight bit of sponge (normal? )

After all of this I am still suffering intermitant brake pedal faliure.

Both times have happend after the brakes haven't been used much, say on the motorway or a long road.

Any ideas!?!? Thanks in advance guys and gals. Oy had the car shy of 1 month .

[RUM4MO]

I would think, that in normal circumstances, the only way that the foot pedal could go down to the floor and still be pressing down on the master cylinder push rod, without any loss of brake fluid, would be if the master cylinder is faulty, for instance its seals are being inverted sometimes in stead of pressurising the fluid to operate the brakes and not allowing the pedal to get down to the floor.

[Ev996sps]

My daughter s tdi 2007 1.9 the same, will be fine for days then travel over halfway to the floor, no fluid loss, but can here air escaping behind pedal every time i press it,,, next day... Fine,,,,???

[Kpb]

I would think, that in normal circumstances, the only way that the foot pedal could go down to the floor and still be pressing down on the master cylinder push rod, without any loss of brake fluid, would be if the master cylinder is faulty, for instance its seals are being inverted sometimes in stead of pressurising the fluid to operate the brakes and not allowing the pedal to get down to the floor.

Yher that's probably the case, Hmmm. Going to have to look into replacing that or at least getting it inspected. I've never had a car with this fault before

[RUM4MO]

Yher that's probably the case, Hmmm. Going to have to look into replacing that or at least getting it inspected. I've never had a car with this fault before

Neither have I - and I don't need it.

@Ev996sps Any hissing air that you hear will just be the brake servo doing its job.

[CarbonChaos]

@Ev996sps Any hissing air that you hear will just be the brake servo doing its job.

Or it could be a faulty brake servo leaking air past its seal and pulling the pedal down ??

[RUM4MO]

@Ev996sps Any hissing air that you hear will just be the brake servo doing its job.

Or it could be a faulty brake servo leaking air past its seal and pulling the pedal down ??

That is a new one on me, any hissing that is due to any failed servo seals will be "weakening" the stored vacuum and make the brake pedal feel harder than it should.

I don't mind being proved wrong and learning something new though, so maybe expand on that theory.(every day can be a school day)

[Jay-Jay]

Ask to bleed again all 4 calipers. Maybe there's some little air bubble trapped somewhere into the circuit.
If possible, bleed the ABS unit too.

[Kpb]

Ask to bleed again all 4 calipers. Maybe there's some little air bubble trapped somewhere into the circuit.
If possible, bleed the ABS unit too.

(Latest update 1)

A week ago I Found a split at the end of the vacume line from the inlet manifold to the booster. I've cut the split section out and re-fitted. The Pedal hasn't gone to the floor since but still feels spongy.

(Update 2)

Stuck it in a garage today. The pedal hasn't gone to the floor since last time but if you pump the pedal when the car is off the pedal does not go hard, It remains slightly soft. According to malcom at my trusted garage he said more than likely dodgy seals in the mater cylinder. Yay. Will know for definate on the next few days

[RUM4MO]

Not that it should be relevant now that you have handed your car into a garage and they think it could be the master cylinder, but, if/when you have a split in that vacuum line, the only effect that you should notice is that the brake could get a lot less servo'd so you would need to stand on the brakes to make them work - though maybe with failing/faulty master cylinder seals, having to stand on the brakes would force the master cylinder pistons down "through" the fluid and not pressurise the fluid and apply the brakes.

[Kpb]

Not that it should be relevant now that you have handed your car into a garage and they think it could be the master cylinder, but, if/when you have a split in that vacuum line, the only effect that you should notice is that the brake could get a lot less servo'd so you would need to stand on the brakes to make them work - though maybe with failing/faulty master cylinder seals, having to stand on the brakes would force the master cylinder pistons down "through" the fluid and not pressurise the fluid and apply the brakes.

Got the car back they said everything looks fine. The bit I fixxed was probably the cause but they couldn't explain why I did it again. But I digress it's working so I'm happy haha.