E38 soft brake pedal doing my head in

[voodoomelon]

I'm popping this question in here as the 7 series questions tend to get no attention; plus this is the same brake system as all other models.

I just bought back my 2000 E38 728i Sport.

It always had a soft brake pedal and 3 years on, its still the same. First 2/3 of pedal travel and basically nothing happens.
So I swapped out the master cylinder last week as I thought I might have damaged it years back, popped in a TRW one.

I pressure bled the whole system when finished and still the same - the first 2/3 of pedal travel is extremely soft and light.

So then did the 2 man bleeding system, had him work the pedal whilst i cracked and tightened the bleed valve. No difference.
I've activated the ABS pump in DIS, no difference.
I've activated the DSC pump too, no difference.

I then pressure bled the whole system again, still no difference. I see no fluid leaks or balooning lines, no evidence of bleeder leaks or brake line joiner leaks.

What in the world can cause this?

Only just now, have I spotted this when the brake pedal is pressed, see the video below:



Is this a normal sound? The air/fluid squelching sound is definitely coming from the ABS module area.

I've also read a report of a bad booster being pressurised before the pedal is even pressed resulting in a soft pedal, thoughts?

[Smokey Bear]

My e39 is similar, I may be wrong but I think it may just be a characteristic of automatics. I don't think my other e39 manual was as spongy. The brake performance is excellent however. It's really just the feel. Once the pedal doesn't collapse under heavy braking I would say it's normal.

[kaveman]

What about fitting braided hoses. And it's cheap enough mod to try out.

[rebel.ranter]

My e39 got softer after changing the pads all round and I could never recover it to the way the pedal used to be.
That included a change of discs and pads some time later too.

You are not the only one that's had this problem, Richard (rpg) from the old forum has the same issue and went to extreme lengths to no avail:
https://www.bimmerforums.com/forum/s...e-pedal-travel

I reckon it has to be related to the DSC unit. But it could be literally anything.

[voodoomelon]

That's such an interesting read RR, he describes everything that's wrong with mine. Difference is, all my parts are stock, as I replaced the pads and discs about 4 years ago when I sold the car and they were all bought from the dealer.

It has big 4 pot Brembo factory calipers on the front and the feeling is ridiculously soft. I've had several people sit in it at this stage and they all agree its not right. There's just no bite at the top end, not even in the middle of the pedal.

One line Richard said in his post resonated with me:

"When I was bleeding the brakes the last time (pressure bleed with GT1 cycling ABS valves) the pedal felt awesome during the bleeding procedure. It was rock solid (no sponginess) and it was right at the top. I remember thinking to myself "Wow, this is going to feel great now" but once I finished the job, lowered the car and took it for a drive, it felt the same as before. "

Mine's JUST the same, I think all is going great and i've finally fixed it, but once the system is sealed again and engine fired up, the pedal just drops near enough to the floor. Compared to my E66 740 it's night and day; I even sat in my mates 525i E39 and his is proper hard too towards the top.

The rear pads on the E38 are down to the wear senors and the fronts aren't too far off either so i've a set of Brembo pads coming for all four corners on Monday. I very much doubt they'll change anything, but fingers crossed.

So frustrating, i've had a few people "in the know" look throw their opinions about and no one is sure. It's such a simple system:

Pedal -> brake booster -> master cylinder -> ABS unit -> line valves -> calipers

Brake booster certainly boosts, master cylinder is brand new, ABS unit could potentially be at fault if that squelching noise isn't normal, all line valves block pressure when tested, no leaks at calipers. The brake hoses are rock hard so no ballooning.

I ran the ABS pump several times for each corner whilst being pressure bled, in conjunction with slapping it on the side to dislodge any potential air, no difference.

So frustrating.

In my eyes, its either the brake booster applying too much boost (if that's possible) or it's the ABS module with bad internal seals or air I can't get out.

[318is-joe]

My E39 is pretty rock solid at 1/4 press great bite, I tried having someone pump the pedal and there is no squelching sound like that coming from the hydraulic block (of the ABS module) - something is not right there I'd imagine. You need to cross check that again with your mates 525i and a few more E39 /38's to be sure, but mine doesn't do that.

Oh and I had a perished rear hose, replaced the four wheel hoses with braided stainless and new fluid - made sweet feck all difference to be honest. I'm on OE pads and discs all round.

[ian.j]

Clamp off one caliper at a time af the flexi hose and pump the pedal with the engine running. Repeat the process on all 4 corners. If it improves at any time you know youve a dodgey caliper.

Only used this process last week on an mk5 golf that had similar symptoms to yours.

Sent from my SM-G920F using Tapatalk

[voodoomelon]

My E39 is pretty rock solid at 1/4 press great bite, I tried having someone pump the pedal and there is no squelching sound like that coming from the hydraulic block (of the ABS module) - something is not right there I'd imagine. You need to cross check that again with your mates 525i and a few more E39 /38's to be sure, but mine doesn't do that.

Thanks for checking that for me, appreciate it. Yep i'll check his E39 for the same noise. If it is the hydraulic block, I wonder if anything can be done to salvage it? Disconnecting it entirely and giving it a good shake, or submerge it entirely in brake fluid?

Clamp off one caliper at a time af the flexi hose and pump the pedal with the engine running. Repeat the process on all 4 corners. If it improves at any time you know youve a dodgey caliper.

Only used this process last week on an mk5 golf that had similar symptoms to yours.

Sent from my SM-G920F using Tapatalk

Thanks I might give this a go when i'm replacing the pads next week. Will also be a good opportunity to examine each caliper.

[Super4]

Hi Voodoo,


What brake pads/discs are in the car? lower quality brake pads can often be the cause of spongy brake pedals and no amount of bleeding will fix the issue.

Are the brake pad retaining clips still in place? If not these are needed as they apply tension and reduce caliper movement in turn reducing pad movement.

Have you tried changing your brake pressure switch/accumulator on the back of the ABS/DSC pump?

Have you inspected the brake booster vacuum hose, elbow and gasket to booster for damage etc?

Have you checked for error codes in the ABS system? If so have you cleared codes and plugged back in after a drive?

Are you bleeding brakes in a Z pattern, starting with the caliper furthest from the brake reservoir?

Does the ABS work if you are to brake hard?

Has the brake master cylinder ran dry at any point in the bleeding process?

[voodoomelon]

What brake pads/discs are in the car? lower quality brake pads can often be the cause of spongy brake pedals and no amount of bleeding will fix the issue.
They're OEM dealer pads and discs.

Are the brake pad retaining clips still in place? If not these are needed as they apply tension and reduce caliper movement in turn reducing pad movement.
Yes they're all in place and in the correct position.

Have you tried changing your brake pressure switch/accumulator on the back of the ABS/DSC pump?
I haven't changed this no, but there was no code for it, would there be if it was bad?

Have you inspected the brake booster vacuum hose, elbow and gasket to booster for damage etc?
Yes no damage to the booster line. Even if there was, surely this would cause a heavier pedal.

Have you checked for error codes in the ABS system? If so have you cleared codes and plugged back in after a drive?
Yes there were no ABS related codes, anything in the history was cleared and no improvement.

Are you bleeding brakes in a Z pattern, starting with the caliper furthest from the brake reservoir?
Yes indeed I am.

Does the ABS work if you are to brake hard?
Can't say I know this for sure as i've only recently bought the car back and its been up on stands for the last few weeks. But the pump activates and all the valves actuate through INPA.

Has the brake master cylinder ran dry at any point in the bleeding process?
No it has not run dry at any point, either through pressure bleeding or the two man process.

I should have new pads arriving tomorrow as they're low all round, so will inspect all calipers then, thanks for the input.
I'm still baffled.