[juld0zer]

You should not apply Brake Quiet to the carrier contact areas (pad ears/ends). Only where the piston touches (paint a circle around the metal piston clip) and where the caliper exterior side clamps on the pads (you could paint the goop on the caliper instead of the pad if you prefer.

What fluid did you use? Do you happen to know who flushed the brake fluid before you overhauled the calipers? Was it the dealer or yourself or an indy shop?

I can relate to the dead first inch feel. I drive heaps of BMWs in my job. You guys are describing it very accurately. It makes modulation difficult because there is no feedback in the form of linear increase in friction or resistance in the pedal. I dont think it's your booster because a booster only assists the pedal down. If it was faulty you would have a high/hard pedal.

Non genuine pads throws another variable into the equation though. Can you borrow some genuine pads to test with? Are they genuine discs also?

[JJaber]

Loaner 16' or 17' X5 M package with 2,500 mile had the same brake pedal feel which makes me feel a little better.

[juld0zer]

X5 has always had a long pedal, helps to remind that you're driving a tank at times haha
But once you reach its consistent friction point it pulls up very well

[rich_mane]

I know this is a bit old, but anyone consider the vacuum pump being the source of the problem? I'm starting to have a similar brake pedal problem where it sometimes takes longer to engage than other times. It has that "on/off" feeling someone described, where it's hard to inch forward without the car lurching.

Since this problem is intermittent, it's hard to pinpoint. I have all new brakes and new bmw fluid with a complete flush. But there are times when the pedal feels normal, and it's very easy to modulate the strength of the brakes. I've also noticed that when the brake pedal is acting normally, the motor is more responsive to lighter amounts of throttle, and overall more responsive in general (AND the exhaust burbles from the MHD flash tune are more intense..) Keeping the throttle light, I can actually get the brake pedal to act normally a majority of the time. Jam the throttle a bit more and build boost though, and I start to feel the problems of a slightly spongey brake pedal and slower motor response afterwards.

Just some food for thought. I'm gonna start testing the vacuum system and trying to find a problem. Probably replace all the lines and maybe go for a new pump if nothing changes.

[Cloud9blue]

Originally Posted by rich_mane

I know this is a bit old, but anyone consider the vacuum pump being the source of the problem? I'm starting to have a similar brake pedal problem where it sometimes takes longer to engage than other times. It has that "on/off" feeling someone described, where it's hard to inch forward without the car lurching.

Since this problem is intermittent, it's hard to pinpoint. I have all new brakes and new bmw fluid with a complete flush. But there are times when the pedal feels normal, and it's very easy to modulate the strength of the brakes. I've also noticed that when the brake pedal is acting normally, the motor is more responsive to lighter amounts of throttle, and overall more responsive in general (AND the exhaust burbles from the MHD flash tune are more intense..) Keeping the throttle light, I can actually get the brake pedal to act normally a majority of the time. Jam the throttle a bit more and build boost though, and I start to feel the problems of a slightly spongey brake pedal and slower motor response afterwards.

Just some food for thought. I'm gonna start testing the vacuum system and trying to find a problem. Probably replace all the lines and maybe go for a new pump if nothing changes.

https://www.realoem.com/bmw/enUS/sho...diagId=34_1483

part #3 tends to leak a bit when the car is a few years old, happens to nearly all BMWs. but that thing is a PIA to replace though since you have to pull out the master cylinder as well and bleed the ABS system afterward (has to be done through coding).

if you had the car since new, you probably noticed that the brake pedal is no longer firm when starting the car first thing in the morning, which means the vacuum system can no longer hold the vacuum for a long time.

but the leak is really really slow typically, so it shouldn't cause any issue when the engine is running.

[juld0zer]

Yes, ive exhausted almost all avenues. Waiting on equipment to arrive for long term vacuum observation. The pump is a pain to replace on these cars. I'm weighing up fitting an electric pump to supplement thr engine pump vs replacing the engine pump. I have access to everything needed to do the job but we'll see.

Rich_mane, your symptoms are a bit odd though. The engine vac pump should generate more vacuum with more engine speed, ie the opposite of a traditional manifold vacuum source. Your exhaust flapper is vacuum controlled also. I'd start with leak checking the plumbing, particularly the tank in the rocker cover as that feeds your wastegate actuator and exhaust flap actuator. The brake booster has its own larger circuit.

Typically, weak vacuum would result in increased pedal effort required to stop the car. Ie. high and hard pedal, not mushy. Mushy or sinking pedal with firm constant pressure could be air or master cylinder.

Cloud9blue, the opposite should be true. If the master to booster gasket was leaking, the pedal would be firm/rock hard in the morning as the vacuum is lost.

[Cloud9blue]

Originally Posted by juld0zer

Cloud9blue, the opposite should be true. If the master to booster gasket was leaking, the pedal would be firm/rock hard in the morning as the vacuum is lost.

ah yes, that's what i meant to type. thanks for correcting that.

[feuer]

Originally Posted by juld0zer

If the master to booster gasket was leaking, the pedal would be firm/rock hard in the morning as the vacuum is lost.

But when engine is running the brake pedal will go to the floor and won't pump the brakes. I have done few M3 master cylinder swaps and on one I overlooked the new seal in the box. Car had zero brakes because air was escaping when you press the brake pedal.

[juld0zer]

Still waiting for my vacuum gauge and fittings to arrive.

In the mean time, i've applied RTV sealant around both vac nipples on the pump. I'll report back if there is any long term benefit

[AD18]

Sorry to bring this back, been trying to find out quantities needed to do a complete brake fluid flush without much success. I've been advised by my local indy that I need 3Litres, does this seem right? Seems like a lot to me...
I'm wanting to change out OEM fluid to rbf600.
Thanks

[Jeff@TopGearSolutions]

Originally Posted by AD18

Sorry to bring this back, been trying to find out quantities needed to do a complete brake fluid flush without much success. I've been advised by my local indy that I need 3Litres, does this seem right? Seems like a lot to me...
I'm wanting to change out OEM fluid to rbf600.
Thanks

For a complete flush with room to spare, 3 liters.

2 Liters would be really tight.

[feuer]

Originally Posted by Jeff@TopGearSolutions

For a complete flush with room to spare, 3 liters.

2 Liters would be really tight.

While ATE Racing Blue was available I alternate between Yellow and Blue in preparation for track season and after 1L the brake fluid color will change. My car is 6MT as well. So 2L would be more than sufficient if you ask me.

[AD18]

Originally Posted by feuer

While ATE Racing Blue was available I alternate between Yellow and Blue in preparation for track season and after 1L the brake fluid color will change. My car is 6MT as well. So 2L would be more than sufficient if you ask me.

Originally Posted by Jeff@TopGearSolutions

For a complete flush with room to spare, 3 liters.

2 Liters would be really tight.

I boiled my brake fluid at a recent track day so will need it all flushed. Just the quantity to buy is what I'm not sure of currently. 2 or 3 litres at the moment, will see if anyone else can chip in to narrow it down otherwise I'll buy 3L to be safe.
Thanks for the replies

[juld0zer]

1.5L is plenty

[PhaseP]

Side note: I found local autozone is carrying low viscosity dot4 Pentosin brand brake fluid in lt containers.

[AD18]

hmm ok not sure how much to buy now...

[bbnks2]

I've noticed this issue as well.

I am not boiling fluid, but sometimes under hard braking the pedal will drop to the floor. Kind of a scary feeling. I bleed my brakes constantly and I've never see air in the lines. I even did the inpa bleed procedure the other day with 2l of fresh rbf600. Still no change. I have all electronic aids coded off too, so that's not a factor...

Sometime brakes are fine, other times the pedal sinks to the floor and then its fine on subsequent presses. Sometimes it even firms up more than normal for subsequent stops after having just gone mushy.

[feuer]

If pedal occasionally goes to the floor without pumping I would suspect faulty slave cylinder at first.

[bbnks2]

Originally Posted by feuer

If pedal occasionally goes to the floor without pumping I would suspect faulty slave cylinder at first.

Just did my clutch and slave was replaced at that time.

It doesn't always happen, but it's been this way since I've owned the car.

[feuer]

Originally Posted by bbnks2

Just did my clutch and slave was replaced at that time.

It doesn't always happen, but it's been this way since I've owned the car.

Sorry I meant master cylinder mounted on the vaccum booster drum. Clutch has master and slave. Brake is only one.

[juld0zer]

Think about it. Between the master cylinder reservoir and the four calipers, you're not going to find a big milk bottle's worth of fluid. You'd be flushing and wasting fluid but up to you - id rather save the money and flush only what is needed.

bbnks2: I had that issue. New DSC valve block and calibration cured it. Basically there is an internal valve which gets sticky and under certain situations where it should hold static line pressure, it instead dumps the pressure as if your wheels were skidding. Which they might - i mainly experienced this issue on panic stops or with sudden increases in foot pressure. But in normal operation, that valve closes again and the pump activates to restore pressure - this is the ABS's function to unlock the skidding wheel and reapply the brake. So your pedal sinks. The master is a mechanical part, if it fails it usually presents symptoms almost all the time rather than intermittently

There is no visible loss of fluid as this all happens internally. There are some documents on the interwebs which go into further detail about ABS operation theory and it might help explain this situation better and go into more detail about which valves are open/closed ans under what conditions.

[bbnks2]

Originally Posted by juld0zer

Think about it. Between the master cylinder reservoir and the four calipers, you're not going to find a big milk bottle's worth of fluid. You'd be flushing and wasting fluid but up to you - id rather save the money and flush only what is needed.

bbnks2: I had that issue. New DSC valve block and calibration cured it. Basically there is an internal valve which gets sticky and under certain situations where it should hold static line pressure, it instead dumps the pressure as if your wheels were skidding. Which they might - i mainly experienced this issue on panic stops or with sudden increases in foot pressure. But in normal operation, that valve closes again and the pump activates to restore pressure - this is the ABS's function to unlock the skidding wheel and reapply the brake. So your pedal sinks. The master is a mechanical part, if it fails it usually presents symptoms almost all the time rather than intermittently

There is no visible loss of fluid as this all happens internally. There are some documents on the interwebs which go into further detail about ABS operation theory and it might help explain this situation better and go into more detail about which valves are open/closed ans under what conditions.

Yeah I don't think anything is actually broken per-se. I think like you said sometimes the DSC just doesn't work the way it's supposed to. I am pretty sure this used to happen on my brand new 128i as well... It's just a quirk. Maybe retrofitting a 1m unit might help for performance driving? Not sure if that's even possible...

This happens under track braking conditions. I was pulling 1g of braking force. Tires could have skidded a bit, but obviously either DSC or ABS glitched somehow and the pedal dropped to the floor.

Moving to more track oriented brake pads will cause the top of the pedal to feel mushy until the pads get hot enough to grip. That's another thing that's gotten worse as I've moved from stock -> stoptech sport -> PFC-08. Each successive pad has had worse cold bite requiring more force the first few stops.