Brake Issue - Sticking Piston...probably

[mmm-five]

I find mine comes back from dealer brake fluid changes like this (only do it for the stamp).

I simply get my indy to re-bleed them the next time I'm in the area (2-3 times a year at least), and the pedal then starts working almost immediately. His uses a compressor as the pressure source rather than a hand-pumped version.

We normally have to push a good litre through the system to get everything out - and it can be a minute or two after the last bubble that we'll see a few more :?

Don't know how BMW did the last bleed though as the nipple was seized when we went to re-bleed them a week later.

 

[Lower]

If you can bring the ABS on, then surely your brakes are working properly? If you've been closing the nipples on each caliper whilst the tube is still full of brake fluid, then by now you must've removed all air in the system? BMW brakes are known for their "progressive" nature, which allows you to modulate the pressure so that you don't apply 100% pressure as soon as your foot touches the pedal. The pedal feel is always softer when the engine is switched off to when the servo is operating when the engine is running. How long have you owned the car? Has it always been like this? Look for leaks.

The friction part of the braking system is fine. I can make the tyres chirp and bring in the ABS ok, but the problem I have is with the long distance the brake pedal travels before any braking effect is felt at all. My E46 M3 brake pedal felt very different and that is how I think these brakes should feel, although I'm happy to be corrected. To confirm, when pressing the M3 pedal, it would travel a short distance before braking effect could be felt and by the time it was braking very hard, the pedal was some where around level with the top half of the throttle pedal, allowing easy heel and toe.

In my Z4M, the brake pedal goes past the throttle pedal when braking with any force so heel and toe is nigh on impossible.

When you say the Z4M pedal is progressive. How progressive is this? I get no braking effect at all for the first 70% of brake pedal travel, then over the last 30% of travel it firms up and the car begins to slow more meaningfully. The travel is over several inches from the top of its travel to about an inch or so below the throttle pedal.

I find that with the engine off, the brake pedal will pump up and become perfectly firm and exactly how I want it. With the engine on, the pedal has no pressure and sinks away as I've described above.

I've had the car since July and I don't recall it being like this on the test drive, nor when I picked it up. Since it's purchase I've been driving it rather 'spiritedly' and I think only after a month or so of ownership I realised that the brake pedal travel was quite long. I'm certain there are no leaks since it uses no fluid and when I stand on the brakes, they do perform well into ABS.

It would be interesting to compare brake pedal feel with another member at some point, just to see if that's how the pedal is supposed to be.

The pedal should go rock solid after a few pumps with the engine off as you release any vacuum left in the servo so that sounds right.

What you describe does sound like trapped air in he system. How much fluid have you pumped through it?

 

[exdos]

Where's this one up to?

 

[bennyboysvuk]

Hello there, sorry I've not followed this one up sooner. I've just not had the time recently.

To answer a question above, I've pushed around half a litre of fluid through the rear bleed nipples and about the same from the fronts.

On Monday I had an interesting conversation with my friendly MOT tester whilst he was MOTing my 1 series and he suggested all the things I've read, PLUS, he said try putting brake line clamps on each brake line near the caliper, then press the brake pedal and gauge how firm it is. If it is completely firm then remove the clamps one by one to see where the problem is. What an excellent idea!

I've purchased 4 brake hose clamps and last night got busy putting them onto each brake hose, by each caliper. I started the car and sure enough the brake pedal was pretty firm. It still travelled a little way and wasn't TVR-Tuscan like in its feel, but it didn't go anything like as far as without the clamps, as you'd expect.

Next, I thought I'd remove the right rear clamp first. I did one and there was a tiny bit more movement in the pedal, which I expected since the piston is now pushing the pad onto the disc. I then removed the rear left clamp and found the same thing with the pedal. All good so far, the rears seem to be behaving themselves and pedal travel is still negligible.

I then removed one of the front clamps and the pedal sunk a lot. I removed the final one and it again sunk a lot more. So it seems there's a problem with the fronts. I whipped the brake bleeder out again and bled another half litre through the fronts, wildly tapping them with a mallet as I went, but alas I saw no bubbles and this was with the pressure bleeder up at 24psi.

I decided to have a close look at the pads and ensure that they aren't too snug in the caliper as I'd noticed they were a little tight when I very first looked at them. However, my indy has done a beautiful job and they move perfectly freely in the calipers. I decided to take them out anyway and examine them on the workbench and to my surprise I could see that they have worn irregularly. i.e. as you look at the pad in place in the caliper, the half of the pad on the lower side has worn much more than the pad material at the top half. When I say much more, I mean only a mm or so. However, having looked this up, I now have hope that this may be the cause of the problem.

I'm off to NY next week (wooo) so can't do anything about it before then, but if I've not spent all my money in NY then when I come back I'll replace the front pads and see if this makes a difference.

Last night I googled a bit more and found one brake troubleshooting guide that said that if you've bled the brakes half to death and the pedal still has a long travel then it could be the pads are worn unevenly. So now my eggs are firmly placed in the one basket. Fingers are crossed and I'll let you know what happens.

Thought I'd add a pic, this is NOT my pad, but it's similar, although mine is slightly less extreme in its wear:

 

[bennyboysvuk]

Quick update...I've found a spare hour and have changed the front pads. They're bedding in currently so I can't get a proper feel for them just yet, but they feel only ever so slightly better, rather than completely different.