Hi,

 

Here comes a really puzzling issue. It all stared with a failed master cylinder on a 2009 Skoda Fabia 1.4 (BXW). After replaced, the brakes engage very quickly when the pedal is depressed just a few mm, and worse, pressure is building up in the lines making the brakes on all wheels drag and eventually lock up. I know this since when loosening the nuts connecting the lines to the MC, fluid is leaking out and the brakes release.

 

I can only see that this can be caused by (1) a faulty MC, (2) a faulty servo or (3) a too long push rod. Since the push rod cannot be adjusted (as far as I know), I first tried with another MC. Exact same result, I then replaced the servo unit. Exact same result.

 

I have tried the following measures to try to get the brakes to release, all without success:

 

1. Remove the break light switch (in case it would hinder the pedal to fully retract)
2. Disconnect the vacuum tube from the servo
3. Loosen the bolts that holds the MC to the servo to create a couple of mm gap between them, simulating adjusting the rod

 

Now, I must admit that I have used aftermarket components, except that the first servo was OEM. For reference, the following products have been used:

 

MC1: Vika 866148 - https://www.autodoc.co.uk/vika/13157615
MC2: LPR 1056 - https://www.autodoc.co.uk/lpr/2360878
Servo1: OEM
Servo2: Borsehung B15996 - https://www.autodoc.co.uk/borsehung/10705705

 

This is honestly driving me crazy. Should I try a OEM MC? But what are the odds that two separate aftermarket MCs have the exact same issue? Any help would be very appreciated.

 

/Mattias
 

 

 

Could the vacuum hose be trapped, kinked or not routed correctly?  I only ask as i had similar once after replacing a turbo to intercooler hose on our octavia that involved the removal of the intake a hose and that caused exactly the same issue of wooden brakes locking on...   

20 minutes ago, skomaz said:

Could the vacuum hose be trapped, kinked or not routed correctly?  I only ask as i had similar once after replacing a turbo to intercooler hose on our octavia that involved the removal of the intake a hose and that caused exactly the same issue of wooden brakes locking on...   

 

I removed the vacuum hose at the servo, letting ambient pressure in without the brakes releasing. Also, the engine then sucked in air through the hose as expected, so I don't think it has anything with to do the vacuum unfortunately.

I believe it's likely to be an ABS module fault, there is a hydraulic pressure accumulator in the module and it sounds like yours may be letting pressure into the system when it should not.

8 hours ago, sepulchrave said:

I believe it's likely to be an ABS module fault, there is a hydraulic pressure accumulator in the module and it sounds like yours may be letting pressure into the system when it should not.

Well, again, when releasing the brake lines at the master cylinder, fluid is leaking out and the brakes are released. To me, this proves that the lines at the MC are in fact pressurized and that the MC does not let this pressure back into the reservoir?

It would be easy enough to temporarily remove a fuse or two to make sure the ABS system was inactive, and see what changes?

Would you like me to see which fuse(s) are relevant?

2 hours ago, garmin said:

Well, again, when releasing the brake lines at the master cylinder, fluid is leaking out and the brakes are released. To me, this proves that the lines at the MC are in fact pressurized and that the MC does not let this pressure back into the reservoir?

Your reasoning and all the things you have done so far make sense to me, like you I believe that the cylinder piston is not able to return fully.

 

Could a spacer or shim of some description have slipped down behind the carpet on removal of the master cylinder or servo. I note that you have loosened the retaining bolts to give some extra clearance.

 

Have you compared the pattern part cylinder with the original?

 

I have only removed the clutch cylinder (far too many times ) on my vehicle and the pushrod ball end is retained in the pedal by a plastic clip which is hard to fully push home when assembled, is there something similar on the brake cylinder?

 

It really sounds like you need to create some clearance somehow to prove our joint belief and then set about finding what is longer or missing.

 

I have had the same on a couple of cars over the years with my bodging/experimentation, a MK1 Escort with a failed direct acting servo which I dismantled to remove the control loading spring so it could be driven as unservoed and a MK1 Scirocco where I was trying to remove the slack from the awfull RHD converted brakes which left the cylinder & servo on the LHS, in both cases I left not enough slack on the pushrod, the brakes released on test but not when everything was at operating temperature.