Soft and spongy brakes - faulty master cylinder?

K

kingwahwah

Member
Hi folks,

This is my first post so please go easy on me :) I've hunted around the forum but none of the existing threads seem to cover my issue or the How To guide I'm looking for.

Anyway, my 2007 2.0l roadster has got a very spongy pedal.

I booked it into a BMW indy last week for a fluid change and some new flexis but they could only bleed 3 of the lines due to a seized NSR bleed nipple.

I'm going to have a go at freeing the nipple off myself with a bit of heat, WD40 and persistence but if I also have no joy, then I'll stick a used NSR caliper on.

First question: Do the rear calipers need a special tool to wind the pistons in to remove them?

I've got a feeling though that the real source of my issue is my master cylinder.

Second question(s): Is changing the brake master cylinder as easy as it looks to be? Is there anything I need to consider with the DSC? Can anyone point me to a How To guide for changing the brake master cylinder?

I'm reasonably handy with fixing stuff and happy to take things on myself whenever I can, but I don't have a garage space for working in...its just on the kerbside and I'm constrained by space/noise/weather, so that's why sometimes it is easier to pay to get it done.

Hope you guys and girls can help!
 
Hi and welcome to the forum. I can answer your first question but not the second. I just had all four callipers off on the weekend for a paint and I didn’t need to wind back the pistons. With the brake pedal fully depressed and blocked, unscrew the callipers from the carrier and just wiggle them off. If stuck on tight, just open the bleed valve a little and draw off some fluid, then wiggle a bit more. Where you have a seized nipple, unscrew the brake line from the calliper to leak out a little fluid and try again. Make sure you clean up any spilt brake fluid and don’t get any on your braking surfaces. :thumbsup:
 
I had this problem once on a VW Jetta. Turned out to be the brake lines. The short rubber ones that connect to the caliper. They get old and weak. If this happens, when you press the brake pedal, the brake hose swells. Definitely check for this condition.

Other than that, it could be the master cylinder, since you already replaced most of the brake fluid.
 
My starting point for spongy brakes after the "professionals" have been at them is a quick bleed all round. Generally a little air has got in where the nipple has been opened. I'd do this before worrying about the master cylinder.

Brake rewind tools are generally only necessary where the handbrake operates on the discs. BMWs have a drum/shoe arrangement in the rear disc.

Saying that, I bought the Laser rewind tool years ago to do someone's Renault and find it great for pushing any pistons back in.
 
Good morning and welcome to the forum.
Definitely worth another bleed first if you can, but until the nipple is freed you won't know if it's all out of course.
If it was spongy before the indy got it then they can't really have made it worse.
Just to let you know that if you do end up going for replacement parts then I have a 2.0i that I'm about to break and when I drove it home all the brakes worked fine. Not sure if the master cylinder is different to the bigger cars?
 
Dont know the history of your car but it pays to bleed the system properly - by that I mean running the Brake Blleed service programe which activates the ABS pump and valves. Without doing this you can have air trapped in the pump valve set.

INPA has such a service programme but means someone has to sit in the cab with a laptop and pump at the same time.

So when you get the bad bleed screw replaced ask if they activated the bleed programme.
 
Huge thanks to everyone for the replies so far - very helpful and very responsive. :D

To clarify my original post - the brakes were spongy before the indy worked on it...that was the reason for booking it in with them, so no inference of them causing the issue....they just didn't fix the problem while they had it in. They almost certainly didn't do any INPA check as I'm sure I would have been charged for it, if they had.

Quick update: I've worked on the n/s rear caliper this evening (the one the indy couldn't bleed due to seized nipple) and I was able to get the nipple open in about 5 minutes by good old-fashioned wire brush, dab of WD40 and a few seconds of a gentle heat gun. That saves me a job and the cost of replacing the caliper at least. Very helpful info for the future though Zebedee and smorris_12. Thanks :thumbsup:

I bled the line through and cleaned out the worst of the old fluid in that NSR line which was a deep shade of red/brown more akin to a pint of best, so definitely needed doing regardless.

Unfortunately, I still have that sinking feeling so makes me think the master cylinder is still the culprit.

I will double check your suggestion though javis20...good shout.

Q: Is changing the Brake Master Cylinder just a case of uncoupling the 2 x DSC pipes, taking the old one off, putting the new one in and refitting the 2 DSC pipes to the new one - followed by a bleed? Or is there some special order or technique that you need to do to ensure you don't damage the DSC? Do you need to bench bleed the MC?

Thanks in advance.
 
Hi Kingwahwah,

Definitely try all before replacing the master cylinder (MC). If you need a MC, make sure to source OE or OEM (ATE).

My 2005 325xi sedan needed a new MC, and I bought an off-brand one because it was the only alternative to OE at 2.5X the price. Visually it looked perfect, but thankfully the Bentley shop manual says they need to be bench-bled before installation. When I bled it, the new MC leaked badly at the gland seal for the shaft. If I had installed it in the car, it could have killed the brake booster. :o
 
Originally you said you had spongy pedal response. Now you reference it as a sinking pedal.
 
Gradually sinking to the floor is definitely master cylinder. Spongy, is either air in the system or worn rubber brake lines. You already bled the system, so not likely air. Hold each of the rubber brake line while someone pulses the brake pedal. If any of the hoses expand, they need to be replaced.
 
I haven't looked closely at the Z breaking setup yet, but changing a master cylinder is generally straightforward and there's no easy way of damaging the DSC unit. Air is going to get in the master cylinder to DSC pipes so you probably will need to run the full DSC bleed program in INPA. (Or force brake fluid back up from a caliper to just fill the top most part of the system. I have no idea if that's feasible but it appeals as a neat idea!)
 
Thanks smorris_12.

I like your theory on the reverse bleed! :D

I'm hoping you're right as the sound of downloading and installing a clean and up to date version of INPA, buying a cable etc. doesn't appeal to me as it sounds like hours of potential frustration. I also use my work laptop for home use and can't install anything on it, so it would mean buying a laptop etc. I'd have to book back into a garage if it came to that.

javis20 - the pedal doesn't sink all the way to the floor...it just has lots of travel before you get the brakes to operate and then a slight but noticeable sink, but that could be some old flexis expanding as you've suggested. If you do a quick double-pump, then the pedal is lovely and hard again.
 
Certainly looking that way javis20.

I worked on the car last night to double-check the OSR line had been bled with fresh fluid by the indy (it had) and also to check whether any of the rear flexi hoses appeared to bulge under pressure (they didn't). The fronts are sporting two brand new flexis so I'm comfortable that they will be fine.

So....looks like a new master cylinder is next port of call to try. Been quoted £91 for one from Fleet Factors which isn't the end of the world.

I feel assured by your message s_morris12 about it being not easy to damage the DSC, but still a little concerned due to cost impact of if it does, so any further advice from someone who has changed one before just to be sure would be welcomed.

My current plan is to put some kind of airtight cap/seal onto the DSC lines as soon as I disconnect them from the master cylinder to minimise air entry as much as possible, and keep fingers crossed. :driving:
 
Good sleuthing, Kingwahwah.

Don't worry too hard about your MC replacement; if you've gotten this far, you're bound to be careful enough. I did one on my 2005 325xi, and they're rather similar. I read through the Bentley manual pretty carefully before I started, and I would recommend doing same to anyone who hasn't done loads of these before (that would be me - only 2 MC installs in my life). Caps of some sort for your lines as you remove them and plenty of rags / paper mats to protect from leaking fluid.

I apparently managed to not get air into the DSC/ABS control module - but I was prepared to do ABS bleed just in case. Good luck, and let us know how it goes!
 
Quick progress update.

Since my last post, I have replaced the master cylinder (bench-bled as best I could), replaced with fresh fluid and re-bled all 4 lines manually using an assistant pumping the pedal. Lots of air bubbles passed through as expected. Before I drove the car, I did a manual check of each jacked up wheel when the pedal was pressed to check each corner was biting the brakes solid - all ok. Afterwards, I went for a road test and the brakes were stopping the car ok but the pedal still felt on the soft side.

Today, I have pressure-bled the system again at all 4 corners using a Gunson Easibleed and there are definitely no more air bubbles in the system coming through.

While I was on, I clamped each flexi off, 1 corner at a time, using a proper pair of flexi clamps (not mould grips) and pressed the pedal to see if that made things feel better. My theory was that if the pedal felt firmer when a caliper was isolated, then it would indicate a problem at the caliper. However, all 4 corners felt the same before and after.

I have taken the car for a test drive and again it is stopping fine but feeling on the soft side.

I did a few emergency stops in a quiet business park and the ABS was doing it's thing ok and again the car stopping, but still the pedal feels on the soft side. I have no ABS warning lights showing.

I'm kind of running out of ideas now :headbang:

I think I'm down to:
1) The master cylinder was the original cause of the issue, but the new one hasn't been fully bled and air remains inside somehow
2) The ABS pump bleed routine needs to be ran with INPA
3) Something else

Any further ideas guys/girls?
 
Have you driven another one? In the nicest way; maybe it’s you and there’s nothing wrong?
 
I haven't driven another Z4 tbh, and I have definitely considered the possibility that it is just me :)

Particularly as my main daily driver is a Jaguar iPace so easy to get spoiled by the feeling of newer cars.

However, a friend of mine who has a 911 had a drive of my Z4 around the office car park before I changed the MC and brought it straight back saying "that needs sorting before something gets worse", so I've at least got a second opinion on it not feeling as it should.

I also had an advisory note from my service from the BMW indy garage back in June that read "Brake feel poor - requires investigation"
 
Next move must be to do an INPA forced ABS bleed to ensure no air remains in the ABS unit. Run the forced bleed and go around the car bleeding each corner.
 
Unless you activate the ABS pump using the BMW service programme you will not be sure to have removed all air from the pump and solenoids etc.as suggested INpA Brake service bleed programme. Yes you have to sit on the cab with a laptop whilst pumping the pedal and bleeding each brake rear left,rear right, front left front right
 
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