Binding brakes

[Wobsta]

Hi,

 

For reference this concerns a Mk1 1u5 Octavia Estate, though I ask as a general query as I imagine braking systems are not that dissimilar across the marque, or indeed across all marques.

 

I have a binding left front brake. It isn't significant, bad enough to wear the pads that side prematurely, maybe 2-3 times as fast, and decrease fuel consumption by 4-6mpg. The wheel is noticeably hotter after drivcing. Dragging would maybe be a better description rather than binding. I hadn't really noticed until my tame MOT tester pointed it out some time ago. I have been trying to track the problem down since.

 

So far, I have fruitlessly changed caliper, flexi hose, bled that side countless times, cleaned and greased pins. Bled master cylinder etc; all the usual culprits that I can think of have been tested/changed to no avail. The master cylinder itself is newish though not OEM (4-5 years maybe - changed it after local garage goosed the original factory part), other than it's all standard.

 

I should add the car stops as expected, the pedal is good and not spongy and I don't notice a pull to one side.

 

I wouldn't say I'm at my wits end as changing brake pads more regularly is hardly a major inconvenience since I DIY, but I don't really want to throw more money at it without a more solid idea of the issue.

 

So, I have been investigating over the last few days. When the brake is dragging, if I crack the bleed nipple open, a little fluid spurts out  (fnarr fnarr). This suggests that there is residual pressure in the brake line, and that the issue is not mechanical. After releasing the pressure, the wheel frees.

 

I had noticed that things improved if I hooked my foot under the brake pedal and pulled up, that things improved (tested many times after coming to a halt at junction, releasing foot brake and remaining stationary, then hooking the brake the car would move). So, on realising this I added a brake pedal return spring. This has improved things greatly and although I haven't lifted that corner to check for bind and that corner is still hotter, it is noticeably better than without the spring.

 

Thoughts?

 

So my mullings. This is a tandem cylinder I believe, and have read reports on other vehicles that return route for brake fluid can be marginally different for each piston in the master cylinder I wondered whether it could be a faulty master cylinder. I have had an issue before where both front discs would bind and resolve with application of hooking, but that was fixed with the current new master cylinder. It seems unlikely that a master cylinder could cause just one to bind?

 

Having messed with the master cylinders (both old discarded and current new), the return spring in them seem quite strong, so I don't see how either of the former scenarios I describe could be the case, though at least one has in the past.

 

Since there is no return spring as standard the brake seems reliant on the servo(is there a spring therein) pushing it back or the master cylinder spring doing the same. IN any case so far, no matter how hard I pull the brake pedal so far, it doesn't seem possible to completely prevent the bind, The servo in the bulkhead appears to be some kind of ball and socket, so there is no potential to adjust clearance. The isn't, that I remember, any means to adjust clearance between servo pushrod and master socket interface and since I don't appear to be able to completely eliminate the bind it would appear that there is at least a very minimal connection.

 

I guess a test I could do is when dragging (after some town driving is best), I could quickly crack the bolts a few turns on the master cylinder to provide a little more clearance and see if that frees the wheel completely. If it does, it would seem to indicate a malfunctioning servo.

 

Can anyone else think of anything to try?

 

 

 

 

 

 

 

 

[Gaz]

I think you’re on the right track, and I’d suspect the master cylinder.

 

Servo surely unlikely to affect the one calliper?

 

If it’s the pedal I’m thinking of, be aware that pulling up on the brake pedal too hard can snap the plastic ‘socket’ in the rear of the pedal and its a right fiddly so and so to get out.

 

Gaz

Edited 1 June, 2018 by V6TDI

[Wobsta]

I doubt the tiny spring or my foot hooking would be enough to snap the plastic bit. Having read some other posts on removing the brake pedal they deem seem to concur with your "fiddly" assessment so I'm planning on leaving that well alone.

 

Two things make me think it's not the master cylinder is that hooking the pedal or the added spring seems to improve matters and that it only affects one front disc. Of course though, if the fault is that there is no clearance between the servo push rod and the master cylinder socket and the master cylinder is slightly activated, then it would make sense that both would bind too, but having had a look at this video:

 

 

which looks like the master cylinder I have, there is a spring between the first and second pistons, and presumably a spring after the second piston which we don't see (no need to remove it I guess to rebuild, but I can't see how the second piston could retract without one ). I can theoretically see how a slight imbalance between those two springs would mean that the brake fluid transfer ports could open a fraction apart from each other. In the big scheme of things this would be unlikely to make any difference provided the master cylinder is free to move back to a completely inactive position. If however the push rod is applying a the tiniest amount of pressure, then perhaps one port could remain closed.

 

I have found some details of a similar problem here:

 

http://www.bba-reman.com/forums/Topic113057.aspx

 

which describes almost perfectly the issues I have. I haven't checked the rear drums on mine, but since they aren't applied anywhere near as much as the front discs are I wouldn't expect to see as noticeable a difference in any case. In this case, two similar issues are described, and servo is diagnosed as the issue.

 

I did also come across this:

 

http://bigcoupe.com/phpBB3/viewtopic.php?t=16351

 

Of course this is on a BMW, but it also reports a faulty master cylinder that causes both discs to bind, but if you have a look at points 11 & 12 the author describes a potential situation discovered by dismantling a master cylinder that could cause a one sided bind.

 

All that said, I think it's the servo being partially activated (and by dint of that the master cylinder too) because, as an update, the spring has now been on for 50 or so miles, and by my temperature assessment using my hand, it seems to have solved the problem though as one might expect the brake pedal action is now a little less than progressive at first application though I'm getting used to it.

 

So back to diagnosis. I'll get by IR thermometer on the front wheels shortly and see how accurate my hand assessment is. Thereafter I'll crack the two master cylinder bolts to see whether it is indeed the servo or the master cylinder. If the latter, I'll likely live with the spring arrangement or maybe bodge something better as replacing the servo on a car this old is rather pointless. It'll likely be going to the scrapyard in the sky in the next year or so in any case as new servos are prohibitively expensive, and the ball ache involved in swapping a scrapper part over in lieu of what I have seems a potential waste time due to unknowns of second hand parts, plus of course it's still a significant amount of time even if it works when application of a spring seems to do the trick.

 

I'll report back in a few days (or weeks if I get lazy and learn to live with the spring).

Edited 1 June, 2018 by Wobsta

[Gaz]

1 hour ago, Wobsta said:

I doubt the tiny spring or my foot hooking would be enough to snap the plastic bit.

 

Spring won't, footsie will.  I found out the hard way, 65 miles from home.  Coz it was FBW I had to keep diagonal upward pressure on the brake pedal (with a Tesco shopping bag - all I had) because it'd kill the throttle otherwise.  '99 B5 Passat by the way.  I even kept a pic of the little tab that snapped:

Rest makes for interesting reading though.

 

Gaz

[Wobsta]

So yeah, IR thermometer located and used, not solved, but much improved. There's a difference of maybe 20-40C between the two fronts. If i recall correctly as well, previously the binding side ran a little hotter too though that could be a false memory, galloping senility and all that. I figure though that the servo at the very least has to be at fault because being that there is no solid connection between the servo push rod and the master cylinder socket, tapping the brake pedal up should have zero effect on the master cylinder if the master cylinder was sticking.

 

It doesn't of course rule out that the master cylinder could have a fault of its own though. So, on to the second test of cracking the master cylinder bolts a turn or two.

Edited 1 June, 2018 by Wobsta