Brake Issue - Sticking Piston...probably

[bennyboysvuk]

This is more a blog of my braking issue that I've been experiencing, but I thought I'd put it out there in case anyone else is experiencing or has experienced anything like it.

I've noticed that when I stop on a hill and just use the footbrake that when I take my foot off the brake the car remains stationary for about a second before the brakes are released and it rolls backwards. I decided to look into this and found that the dust boot on the front right brake piston isn't quite in position properly, so I assume that dirt has got onto the seal of the piston and perhaps the piston has corroded slightly, thus causing the brake stickiness.

The other issue that I'm finding is that the brake pedal has a very long travel before anything happens. I have to push the pedal what feels like more than 2 or 3 inches before any braking occurs. By comparison, my 120Ds brake pedal feel is immeasurably better and firms up after perhaps less than an inch or so of travel.

I was going to rebuild the caliper myself, but after reading into it, I dislike the messiness of the job and all the time consuming brake bleeding that goes with it and so I've booked it in with BMW specialist Lee Shannon (in Bentley), who is going to rebuild the caliper and fit a new seal and dust boot. I hope that the piston itself is unmarked, otherwise that will need replacing and apparently BMW don't sell the piston on its own, only as a whole with the caliper. Getting a second hand caliper might be an option though.

I'll update the post once the work is done and hopefully, normal, decent brake pedal feel will have returned.

 

[Jembo]

When was the last time you completely bled the system, because with brake pistons all round you should feel contact after a few mm... increasing in ferocity the harder you press. It's recommended to change Brake fluid every 2 years & if the brake fluid has boiled at any time will require changing even earlier.

 

[srhutch]

You do know the Z4 E85/86 has hill assist where on a hill the car will not roll back for a second or two when you release the foot brake so long as the car is in gear.

 

[pilchardthecat]

Hill Start Assist, very useful feature

The brakes have a lot more feel than the 1 series, and they are servo assisted to a lesser degree. You get used to it - the sharpness you are missing from the 1 series is a result of over assistance

 

[benlumley]

You do know the Z4 E85/86 has hill assist where on a hill the car will not roll back for a second or two when you release the foot brake so long as the car is in gear.

+1 - the releasing sounds like that

 

[mad4slalom]

the pedal on my m is almost oversensitive at the top, so that maybe a caliper problem, re hill start assist, sorry to hijack but do all zeds have it or was it an option ?

 

[srhutch]

the pedal on my m is almost oversensitive at the top, so that maybe a caliper problem, re hill start assist, sorry to hijack but do all zeds have it or was it an option ?

All facelifts do, not sure about pre facelift though.

 

[pilchardthecat]

all Ms have hill start assist and brake drying

 

[andysat]

Deffo sounds like hill assist.

 

[bennyboysvuk]

Ah, I'm relieved to hear that it's hill-assist. I thought it was rather pronounced for a brake problem.

I bought the car in July so I'm not sure when the brake fluid was last done without looking it up.

My Indy will bleed the system though and hopefully decent brake pedal feel will return.

Incidentally, last night I checked the near side front caliper and the dust boot wasn't correctly fitted on that side either. Is this a common problem with Z4Ms or has my Z been exposed to poor workmanship in the past?

Thanks for all your responses.

 

[bennyboysvuk]

The brakes have a lot more feel than the 1 series, and they are servo assisted to a lesser degree. You get used to it - the sharpness you are missing from the 1 series is a result of over assistance

They do when there is bite at the top of the pedal travel, but the pedal goes so far down on braking that I can't even heel and toe. If I pressed the brake pedal 1 inch in the 1, the car would stop. In the M the pads haven't touched the disc with the same pedal movement. It's definitely a problem with my car though and I'm sure will be back to normal once fettled and bled.

 

[bennyboysvuk]

Yesterday my car went in to have the dust boots around the front calipers rebuilt, since they had popped out of their seating a little. The brake fluid was also flushed through using a Sealey pressure bleeder.

Sadly, all of this didn't correct my long brake pedal problem that I have currently. Here's a rather unexciting video of how far my brake pedal goes before there's any resistance: http://s29.photobucket.com/albums/c299/bennyboysvuk/?action=view&current=IMG_0765_zps06307c38.mp4 Heel and toe is impossible at the moment since the brake pedal is the wrong side of the throttle pedal by the time any braking force is present.

Next thing on my list to try to get some brake pedal feel is by doing manual bleeding, employing my wife to do the pedal pushing, following this method from the mad Russian M3 site:

Have the helper press the brake pedal down to the floor again and hold it there.
Open the bleed valve. Make sure your helper does NOT release the pedal as that will suck air back into the system!
The system will lose pressure and the pedal will go all the way to the floor.
Once fluid stops flowing close the valve and then tell the helper to release the pedal.
Repeat this step at least 3 times - or till no more bubbles are seen.
Make sure that your brake fluid level does not drop below minimum in the reservoir or you will suck air into the system - add fluid as required to keep it topped off.
Repeat for the rest of the brake calipers (including Step 8 above for each).
Once you have done this for all of the calipers your pedal should be VERY firm - this is the goal - if its mushy you still have air in the system - repeat the process or have a professional do it for you.

 

[deka]

I am surprised that the flushing of the brake system did not clear any air trapped in the system?, has the fluid been changed?... you can get some bleed tools that will allow you to do it by yourself

 

[sammyz]

Bleeding the system should have sorted out any 'mushiness' in the brakes. Are the brake pads worn? Have you checked the hoses for any bulging? Unlikely I know.

Failing that and assuming it was bled properly by the garage then you will need to get it to a BMW specialist for an opinion I think.

 

[bennyboysvuk]

Thanks for your replies folks.

The brake pads are a little worn, but they're thousands of miles from replacement yet and I've never experienced a long pedal like it just from worn pads.

I've read that lots of people in the US have had this kind of issue and they have got around it by bleeding the brakes all round with a pressure bleeder and then going out and hitting the brakes hard enough to trigger ABS in quite a big way. Then they've used the pressure bleeder for a second time and it's cured the issue. So I've ordered a Sealey VS820 and some Castrol brake fluid and will give that a go first.

 

[exdos]

You'll get rid of all the air in the brake lines with a pressure bleeder, because it forces fluid through the brake line from the reservoir continuously, bubbles and all. Whereas, pressing the brake pedal to bleed only forces a limited amount of fluid through the pipe at each press of the pedal and bubbles of air can get compressed.

 

[bennyboysvuk]

Unfortunately, it's still not fixed, but I think it's ever so slightly better.

Last night I used my new pressure bleeder on the car and bled all four calipers, then went for a drive, used the abs+dsc several times and then re-bled the system again. I believe I saw a few tiny bubbles come from the calipers, but it's difficult to know if this is just the air trapped in the bleed nipple coming out.

The result is that the long pedal I think has improved ever so slightly, but I'm slightly worried that this is a placebo effect. I think the next thing is to try a manual bleed with my wife doing the brake pedal pushing. I'll update the thread once that's done. Fingers well and truly crossed that it improves things.

 

[exdos]

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.

 

[bennyboysvuk]

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.

 

[exdos]

You're obviously familiar with BMW brakes so what your experiencing must be abnormal.

I'm guessing here, but I'd take a close look at the servo side of things. If you locate the servo behind the brake fluid reservoir and trace the black plastic pipe from the servo to the air-intake system on the opposite side of the engine bay, this pipe joins a "Y" connector. If you switch the engine on and look/listen for leaks around this junction and the other smaller pipes that form part of the same network, you might find a disconnected or leaking pipe which is preventing the servo from working properly?

Failing that get a garage to examine it for you.