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Discussion Starter · #1 ·
Please can anyone advice on potential or likely causes and end my nightmare with my brakes.


The car has been to a bmw M series specialist and also in at the main dealers and everyone so far who has looked at the car is scratching their heads.


so about 7 months ago I wanted bmw to fit stoptech stainless steel brake hoses during a standard brake fluid service. they refused so I got a competent garage to do the work instead. during this the rear brake lines were found to be corroded and they were replaced up to the join near the gearbox.


This is where the hell has started.


after having the car back after a few days the pedal crept more and more. after taking the car back and bleeding the brakes this solved the problem but not for long (same effects). the mechanic thought that it was unfortunate timing and thought the symptoms pointed towards the master cylinder. this was replaced but the symptoms remained. after swapping the hoses back to standard bmw ones a brake servo unit was replaced to rule this out. the car was then but into a bmw m specialist who then replaced the master cylinder again incase it was faulty or wrongly fitted.


they then questioned the abs/dsc unit. the unit was removed and sent to a specialist company who bench tested. long story short all was good:


Pass. Good communication protocols and live data readings for all wheel speed sensors.
Pressure test of ABS block passed, block holds pressure and has good flow through valves, no blockages or leaks. Motor activation test passed.


The car is now in at main dealer who want to replace rear lines back to standard (this includes removal of fuel tank and rear axel carrier as the lines are pre-bent, whereas the person who replaced first time didn't have to do as the line was on a reel and not prebent). they also want to replace front and rear discs and pads at a cost of over £4100.
this is without looking into the dsc unit that could add another £2000 if required.


before the abs unit was removed there were never any lights or fault codes.
after the unit was removed the car was started to be drove onto a trailer to trailer it to the m specialist to refit the unit. obviously this lit up a lot of fault codes as the unit was not fitted.


once refitted the codes that remain are:


5DD0 - HYDRAULIC ADJUSTMENT, DSC UNIT (VALVES)
5DD2 - HYDRAULIC ADJUSTMENT, DSC UNIT (VALVES)


I am told by both the m specialist and bmw main dealer that these codes will not clear when trying to recalibrate the dsc unit.


does anyone know what would cause the calibration to fail bearing in mind that this unit has supposedly been tested and was never showing these codes before?


that is the electrical side of my problems...


now the mechanical side of the symptoms are that I loose all brake pressure to the rear wheels.
each time I bleed the brakes I get a solid pedal and brakes are good including the rears. this lasts for a few hours of driving but then pedal drops to floor and I loose just the rear brakes again and can turn them by hand with the brakes applied.
we have found on multiple occasions that air is getting into the system but everyone is confused as to how. the whole system has been checked over visually and there appears to be no leaks and there is no fluid loss.


any thoughts would be great as im on the verge of giving up on this amazing machine due to a poxy brake problem.
 

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Discussion Starter · #4 ·
Yes has has had:
?1 x servo unit
?2 x master cylinder
?all around brake hoses twice
?tested abs/dsc

The thing I don't understand is that it's both rear calipers that loose pressure and air is introduced into the system with no fluid leaks. Surely air must replace fluid ie a leak?
Has anyone ever had or heard of this in any car where there are no leaks but loss of pedal?

Would I be right in assuming the following...

?The master cylinder is not at fault as once the brakes are bled both front and rear brakes are very good and the pedal is hard.
?the servo has been replaced but would not be able to cause the problem I'm having (pedal to the floor + no rear brakes)
?the abs/dsc module has no leaks and In a normal state just acts as 2 splitters, 1 line into 2 fronts and 1 line into 2 rears.
?from here it's only lines and calipers which have no leaks.

None of it makes sense.
 

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Discussion Starter · #5 ·
Yes jcolley the brakes have been bled uning dis function to activate the pump. It's been into Bmw main dealer and also a specialist with dis and gt1. The last time the brakes were really good after around 3L bled through each corner to ensure there was no chance of air remaining. Following day after driving great for a few hours, pedal starts to travel further and further each time brakes are used until pedal goes to the floor. Car still stops but when investigated it's only front brakes that are being applied, if same as usual lines are back with air in them...
 

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Is it even remotely possible that the calipers have been reinstalled upside down? They'd have to be swapped right to left. If so they'll store air forever. I did this once on another car and it acted just as you describe.
 

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...we have found on multiple occasions that air is getting into the system but everyone is confused as to how. the whole system has been checked over visually and there appears to be no leaks and there is no fluid loss.
I don't know if this is possible on our cars, but something to check:

Brake fluid from the master cylinder can leak into the brake booster, drawn into the intake and burned by the engine.
 

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Discussion Starter · #9 ·
Thanks Clyde, ive had a new brake booster/servo fitted after the symptoms started.
With the scenario you've said about (would there by any signs of visable leak or all inside the part? There's never been major fluid loss from the resivior.
 

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...With the scenario you've said about (would there by any signs of visable leak or all inside the part? There's never been major fluid loss from the resivior.
I have never personally had this problem, so you might want to just search for "brake booster leak" or similar.

If there is brake fluid in the vacuum line to the booster, a leak into the booster might be the cause of your problem.
Your leak is slow, so there might be no evidence of brake fluid.
 
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