Brand new brake cylinder doesn't hold pressure

[sdutton007]

My 2012 A3 2.0TDI has been off the road for well over 2 years now with a weird brake fault where the brake pedal won't go firm, no matter how many times it's bled or what parts have been replaced.

I've gone to the extreme of blocking off the brake pipes at the ABS pump and bleeding it fully again - the pedal still doesn't go firm.

So now I've blocked both brake pipes off at the Master Brake Cylinder, bled it again - and it still doesn't hold pressure. This is the THIRD master brake cylinder I've put on this car to try fixing it, and it's brand new from Audi.

In my mind, the only way that the pedal wouldn't be firm in this situation is if the internal seal has failed but the chances of having 3 different master brake cylinders with the same problem, including a brand new Audi one, seems statistically impossible.

Can anyone make any sense of this?

 

[NHN]

Did you change the servo?

 

[sdutton007]

Did you change the servo?

No, but the engine has been off the entire time so the servo shouldn't be affecting anything.

Although if the servo wasn't such a nightmare to change, I'd do it just to be 100% sure.

 

[sdutton007]

Update:

I bench-bled one of my old master cylinders last night and got it to the point where I physically can't compress the piston using both hands - while this isn't as much pressure as using a foot pedal, I'm thinking that one *might* be OK. What's the betting it'll stop working as soon as it goes near the car?

I also spent another half hour today bleeding the brand new master brake cylinder with the ports blocked off, and the pedal is still as soft as ever.

 

[NHN]

No, but the engine has been off the entire time so the servo shouldn't be affecting anything.

Although if the servo wasn't such a nightmare to change, I'd do it just to be 100% sure.

The likeliness of it being multiple MC's are a stretch, hence why I suggested servo, not much more can be tbh if it's not holding pressure.

 

[sdutton007]

I believe I've found the issue - the servo pushrod was sitting in the wrong place (right at the top instead of in the middle) and appears to have been scraping against the MBC piston (which has damaged the brand new MBC) instead of pushing the piston in.

That would explain why the pedal never gets hard.

Very strange though, I've never heard of this happening even after a lot of research and even getting a mechanic out for a few hours.

This time, I'm going to put a weight on the pedal and fit the MBC over the pushrod to make sure this doesn't happen again.

 

[NHN]

Like I said, servo.

 

[sdutton007]

Like I said, servo.

Servo is fine, thankfully. I really didn't want to have to replace that!

 

[sdutton007]

The car is now all back together, brake pedal is rock hard and the servo is working perfectly! I'm so happy!

The brake lights are stuck on, even with 2 different brake light switches. Hopefully it's just a fuse though, will check when it's daylight.

Hope this thread helps someone in the future, and they don't have to spend 2.5 years trying to fix it like I did

 

[LEE69]

What was it?

 

[sdutton007]

What was it?

The pushrod that goes from the pedal to the MBC was sitting in the wrong place (right at the top instead of in the middle) and appears to have been scraping against the MBC piston (which has damaged the brand new MBC) instead of pushing the piston in.

 

[LEE69]

The pushrod that goes from the pedal to the MBC was sitting in the wrong place (right at the top instead of in the middle) and appears to have been scraping against the MBC piston (which has damaged the brand new MBC) instead of pushing the piston in.

Ahh got ya, fair play for sussing it out