greg123
07-12-2008, 03:20 PM
Bit of a long story. A guy from another forum took his car with brakes working fine to the MOT. When the guy tested them on the rollers, the rear weel cylinder blew (it had corroded against the backplate and been pushed out of line and the piston flew out of it). It thus failed the MOT and he ran the master cylinder dry.
He put a new wheel cylinder on and bled it with a pressure bleeder. However he couldn't get a good pedal, it only worked on the bottom 1/4 of the pedal, needed pumping up and sank rapidly. He then thought it was the master cylinder as well having had the seals damaged by the sunk pedal & running the pistons down the bore where they don't normally run and crosion buils up, tearing the seals (I have seen this a number of times).
So he brouht it to me with a master cylinder asking for a full bleed inc the ABS block with VCDS and fit the new master cylinder. I put about 3 litres of fluid though the pressure bleeder & ran the ABS bleeding procedure 3 times following the VCDS prompts eg opening the front bleed screws, pumping the pedal, etc. All fluid coming out was new and no air, total flush and major bleeding session!
The pedal was fair, but not great, till I got the enine running again. Then when I backed it out and put my foot on the brakes, it sunk down to the floor again. Not as bad as when it came to me, but still bad.
Anyhow by now I'd pressure washed the car underneath and couldn't check for leaks other than the new master cylinder connections which were bone dry and he had to take it back to get home. I thought there is another problem here not diagnosed and the guess on the master cylinder is wrong.
This is where it gets strange. He phoned me that night after driving to two town. He said that the pedal had got better and better and by the time he got to is first pickup, it was 100% perfect and remained so. I didn't understand it, but great.
The next day after being stood overnight, it was also 100%. But.... later that day they started to get worse and by the evening they were back to square one, sinking pedal only working on the bottom 1/4 of the travel.
No loss of fluid, no leaks that can be seen easily (I advised him to get the car dry and up on a 2 post lift while someone worked the brakes - this has not been done yet) but this very poor pedal which seemingly fixed it's self but only for 24 hours.
I'm now inclined to think that perhaps valve is sticking in the ABS, interupted by the initial bleeding when he changed the wheel cylinder and is letting the pedal sink somehow. However I don't have an ABS hydraulic schematic and don't know if this is possible or what the fix is, other than a new new one. I have heard of ABS blocks throwing a wobbly after pressure bleeding (yet VAG specify pressure bleeding on these systems, you have to in order to bleed the ABS block in fact along with using VCDS).
Either that, or he has got a leak somewhere else. Or the new, and old, master cylinders are faulty. Or there is air in the system still, but how, and how can the brakes be perfect and rock solid for 24 hours when there is enough air in to sink the pedal to the floor the next day? It doesn't add up.
All of the above are a bit far fetched I think so I was hoping someone here had dealt with a similar problem so I can get some more opinions. It's actually on an Octavia disk/drum with abs setup but as that forum is very quiet and it's a golf 4 platform I'm posting it here.
Thoughts appreciated,
Greg.
He put a new wheel cylinder on and bled it with a pressure bleeder. However he couldn't get a good pedal, it only worked on the bottom 1/4 of the pedal, needed pumping up and sank rapidly. He then thought it was the master cylinder as well having had the seals damaged by the sunk pedal & running the pistons down the bore where they don't normally run and crosion buils up, tearing the seals (I have seen this a number of times).
So he brouht it to me with a master cylinder asking for a full bleed inc the ABS block with VCDS and fit the new master cylinder. I put about 3 litres of fluid though the pressure bleeder & ran the ABS bleeding procedure 3 times following the VCDS prompts eg opening the front bleed screws, pumping the pedal, etc. All fluid coming out was new and no air, total flush and major bleeding session!
The pedal was fair, but not great, till I got the enine running again. Then when I backed it out and put my foot on the brakes, it sunk down to the floor again. Not as bad as when it came to me, but still bad.
Anyhow by now I'd pressure washed the car underneath and couldn't check for leaks other than the new master cylinder connections which were bone dry and he had to take it back to get home. I thought there is another problem here not diagnosed and the guess on the master cylinder is wrong.
This is where it gets strange. He phoned me that night after driving to two town. He said that the pedal had got better and better and by the time he got to is first pickup, it was 100% perfect and remained so. I didn't understand it, but great.
The next day after being stood overnight, it was also 100%. But.... later that day they started to get worse and by the evening they were back to square one, sinking pedal only working on the bottom 1/4 of the travel.
No loss of fluid, no leaks that can be seen easily (I advised him to get the car dry and up on a 2 post lift while someone worked the brakes - this has not been done yet) but this very poor pedal which seemingly fixed it's self but only for 24 hours.
I'm now inclined to think that perhaps valve is sticking in the ABS, interupted by the initial bleeding when he changed the wheel cylinder and is letting the pedal sink somehow. However I don't have an ABS hydraulic schematic and don't know if this is possible or what the fix is, other than a new new one. I have heard of ABS blocks throwing a wobbly after pressure bleeding (yet VAG specify pressure bleeding on these systems, you have to in order to bleed the ABS block in fact along with using VCDS).
Either that, or he has got a leak somewhere else. Or the new, and old, master cylinders are faulty. Or there is air in the system still, but how, and how can the brakes be perfect and rock solid for 24 hours when there is enough air in to sink the pedal to the floor the next day? It doesn't add up.
All of the above are a bit far fetched I think so I was hoping someone here had dealt with a similar problem so I can get some more opinions. It's actually on an Octavia disk/drum with abs setup but as that forum is very quiet and it's a golf 4 platform I'm posting it here.
Thoughts appreciated,
Greg.