Persistent Soft Brake Pedal

[bugg]

When I bought my E90 used, it came with calipers painted weird yellow. As I was coming up on needing new rotors and pads, I decided to source a set of used calipers and replace those too.

Last week, I replaced:
- outer brake lines (Bimmerworld braided, gave up on the inners)
- rotors (FCP OEM)
- calipers (used OEM)
- pads (Akebono Ceramic).

The calipers had one dust boot that needed replacing but otherwise appeared in good condition. I did not rebuild them completely. Brake pedal was halfway pressed during the install and I didn't lose much fluid.

Brake pedal was immediately soft after install and bleed w/Motive Power Bleeder. "No problem!" I thought, and bled the DSC pump w/ISTA. It got better, but was still not driveable. I had a reputable (and close by!) German indie take a look today. They bled w/5 gallons of brake fluid, and bled the ABS pump multiple times without seeing bubbles. There was a marginal (<10%) improvement. They're stumped, and are now referring me to a master tech @ BMW Seattle that they work with.

Any suggestions on what I could troubleshoot? Maybe deansbimmer might know something.

There doesn't appear to be a fluid leak, the possibilities as I see it are:
- Air ingress secondary to failure in the calipers somewhere.
- Air ingress secondary to improper install of the lines. Maybe washers placed incorrectly?
- persistent bubble somewhere.

UPDATE:
Found out about the "DSC Test Plan aka DSC Unit Adjustment" in ISTA. Going to give that a try, when I can. My battery voltage was already low from the ABS bleeds, and then the parasitic drain from my dashcam fried my FRM module. Gotta wait 'til Monday to take it into a local tuning shop that reflashed mine last time.

[Redd]

So you replaced your original (yellow) calipers with a set of used ones and had soft pedal? Have u tried going back to the old calipers?

[Gossypiboma]

This might help with your issue. Worth a try at least.

https://m.youtube.com/watch?v=5jkJQ9DiLuo&feature=youtu.be

[bugg]

Haven't tried going back to the old calipers yet. I'm going to give the DSC unit adjustment a try, after fixing my FRM module. Can't do it in my driveway as I have a sloped driveway and it needs a level surface, but I don't want to drive around without taillights. Hopefully the DSC adjustment fixes it and I don't need to go through a caliper reinstall.

[AK M3]

I had the same issue with the soft pedal after installing the akebono ceramics. The initial isn’t as aggressive as the OEM pads and I think that’s what contributes to the softer pedal. Just something I’ve gotten used to by now.

[bugg]

Originally Posted by AK M3

I had the same issue with the soft pedal after installing the akebono ceramics. The initial isn’t as aggressive as the OEM pads and I think that’s what contributes to the softer pedal. Just something I’ve gotten used to by now.

I think that's part of it. The initial bed-in was kind of scary, but there's reasonable friction now. However the pedal still has very little resistance until the last 20-30% of travel. Logically, it feels like there's a smidge of air that's getting compressed down but maybe there's some DSC recalibration sorcery at play here.

[Ximian]

DSC unit adjustment would help, but it also sounds like your brake cylinder and brake booster might be leaking. Remove the cover under the steering wheel and check the brake booster for leaks. Another sign is a lot of brake fluid under the brake fluid reservoir.

[bugg]

Originally Posted by Ximian

DSC unit adjustment would help, but it also sounds like your brake cylinder and brake booster might be leaking. Remove the cover under the steering wheel and check the brake booster for leaks. Another sign is a lot of brake fluid under the brake fluid reservoir.

Didn't check under the reservoir, but brake booster looks dry. Only thing on there was a thin film of old grease and dust.

Taking it in tomorrow in the AM to see the dealership master tech the indie referred me to.

[Ximian]

Originally Posted by bugg

Didn't check under the reservoir, but brake booster looks dry. Only thing on there was a thin film of old grease and dust.

Taking it in tomorrow in the AM to see the dealership master tech the indie referred me to.

Clean it and check again. I'm willing to bet the thin film comes back after a few days of driving.

Also, how often is your brake vacuum pump running? Almost after every time you fully depress the brake pedal?

[Redd]

Also check your DSC pump unit itself. I troubleshoot a soft pedal problem for 2 years which in the end turned out to be a small leak at the DSC pump. The reason I didn't mention this earlier was cos you had just changed calipers so I thought that might be the cause.

[bugg]

Car's still at the dealer right now, no updates yet. The brake vacuum pump and DSC pump are good suggestions, but I'm unable to check now. I wonder if a bit of fluid might have dripped down the outside and shorted the vacuum sensor - I definitely did spill a bit of brake fluid (I got tired and sloppy about 7 hours in).

[Tommysalami]

Originally Posted by bugg

Car's still at the dealer right now, no updates yet. The brake vacuum pump and DSC pump are good suggestions, but I'm unable to check now. I wonder if a bit of fluid might have dripped down the outside and shorted the vacuum sensor - I definitely did spill a bit of brake fluid (I got tired and sloppy about 7 hours in).

You'll get constant vacuum pump running and limp mode if the vacuum sensor got shorted out

[Redd]

Yeah and vacuum sensor won't cause soft brake pedal.

[bugg]

Well Hanlon's Razor wins, again. My tired, dumb ass installed the rears on the wrong sides so as made obvious by the above photo, air gets trapped in single-nipple calipers and can't be bled from above the nipple.

So, so humbled lmao

THREAD CLOSED and we never speak of this again

[Tommysalami]

Originally Posted by bugg

Well Hanlon's Razor wins, again. My tired, dumb ass installed the rears on the wrong sides so as made obvious by the above photo, air gets trapped in single-nipple calipers and can't be bled from above the nipple.

So, so humbled lmao

THREAD CLOSED and we never speak of this again

Hah, that's probably the best issue you could have wished for

[bugg]

Originally Posted by Tommysalami

Hah, that's probably the best issue you could have wished for

At least my lines are now really, really really clean.

[spammysammich]

I’m shocked that nobody picked up on that until now.

[RocketyMan]

Originally Posted by bugg

Well Hanlon's Razor wins, again. My tired, dumb ass installed the rears on the wrong sides so as made obvious by the above photo, air gets trapped in single-nipple calipers and can't be bled from above the nipple.

So, so humbled lmao

THREAD CLOSED and we never speak of this again

Appreciate you reporting back on this! This may seem so simple now, but can often get over-looked when changing out calipers. Now I know, too!

I love learning about stuff like this.