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2Addicts | BMW 2-Series forum Technical Topics Suspension | Chassis | Brakes managed to get air into the master cylinder when bleeding....

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      05-27-2020, 02:17 PM   #1
vexingv
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managed to get air into the master cylinder when bleeding....

i changed to stainless lines and went about bleeding the lines. i bought a Motive pressure bleeder. I wanted to minimize some of the mess of cleaning the bottle (i can't seem to find denatured alcohol anywhere locally) and decided to use the system as a pressurizer. I filled the brake fluid reservoir with new fluid and pressurized the system. then started bleeding from the rear passenger side. i kept an eye on both the pressure gauge and the reservoir.

now, it's a hot and sunny day in the garage so i used a flashlight to help me gauge the reservoir tank level. but before i knew it, i started to hear gurgling sounds from the catch bottle. i thought, wow there must be a lot of air in lines....

but in actuality, i had emptied out the reservoir as the sunlight made it very hard for me to gauge the volume in the reservoir and it actually emptied and so i was just pushing air through the system.

i then closed up the bleeder valve, filled the brake fluid reservoir tank , then filled the Motive tank with the remainder plus a second bottle of Pentosin brake fluid.

then i bled the system again.

i know i messed up and really should have just filled the tank to start, which would have prevented this problem.

anyway....
i've tried the brakes, (while the car is still on jacks as i still need to replace my tensions struts as part of this project) they still feel firm. although maybe have some slight travel at the very end if i hold on for a long time (although i think that's what i've always experienced before).

what should i do?
1.
bring to a shop for a more thorough flush that includes cycling ABS? (probably will be bringing car in for an alignment after replacing my tension struts so i could probably have this done at the same time)
2.
bench bleed the master cylinder? i don't really know the process and whether i could (or want to) do it myself, but this topic did come up on searches for "air in master cylinder"
3. pressure bleed the system again with another bottle of brake fluid?
4. don't worry about it. this is a daily driver, although some spirited driving on freeways, this is not a track toy so i don't expect/need every bit of performance
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      05-27-2020, 03:10 PM   #2
Maynard
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1. yes, definitely. And put in factory fluid if you aren't tracking it (or is Pentosin the OEM?).
2. only if you want to drag this out before paying for #1 plus paying them to refit the master cylinder (and redoing all the paint that will get eaten by brake fluid)
3. probably irrelevant - if there is still air in there, whatever you are doing isn't clearing it.
4. Not this. If you were satisfied with poorly functioning or unreliable brakes you wouldn't have tried bleeding them in the first place. And I'd much rather lose brakes on track vs. on the street.
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      05-27-2020, 05:12 PM   #3
msendit
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Quote:
Originally Posted by vexingv View Post
what should i do?
1.
bring to a shop for a more thorough flush that includes cycling ABS? (probably will be bringing car in for an alignment after replacing my tension struts so i could probably have this done at the same time)
2.
bench bleed the master cylinder? i don't really know the process and whether i could (or want to) do it myself, but this topic did come up on searches for "air in master cylinder"
3. pressure bleed the system again with another bottle of brake fluid?
4. don't worry about it. this is a daily driver, although some spirited driving on freeways, this is not a track toy so i don't expect/need every bit of performance
Nothing to worry about, we've all done that. It's just some air, that's what flushing is for. Plus, when you had the lines off the system was all air anyway.

I'd go with #3 -- you already have the pressure bleeder and want to learn to DIY. What's another bottle of brake fluid to learn a new skill :-)

Annoyingly I've found that a full flush on our cars with stock brakes is juuuust a bit over 1L of fluid. Which with the fluid I use is 2 bottles and a hair. Which of course means a 3rd bottle dangling around waiting for a partial bleed...
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      05-27-2020, 05:38 PM   #4
Liandri
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Just rebleed it. I use a mityvac pressure bleeder most days at work, though I've never let it blow air through, all you need to do is just rebleed the whole thing. If the pedal is firm still, there can't be a whole lot of air, so rebleed the RR line first and when no air is coming out proceed to the LR then the fronts. Remember to be careful not to get brake fluid on the caliper paint. I would never bring it to a shop in this instance as you already have the tools to fix it yourself and you care the most about your car. Also bench bleeding is not applicable here, definitely don't do that. Bench bleeding is for new in box masters and I can't even tell you the last time I or any other techs around me did it. Pressure bleeders negate the need for any other type of bleeding, mostly. Fwiw I've pressure bled my m235i twice now without issue, and did not go into the abs cycle procedure. I don't think it's necessary on a street car to get every micro milliliter out of the pump and TBH I'm not sure anyone on here knows how much is actually in there anyway, but I would love to be corrected on this
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      05-27-2020, 06:22 PM   #5
dradernh
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Quote:
Originally Posted by msendit View Post
What's another bottle of brake fluid to learn a new skill :-)
I believe that is real wisdom being offered.

Because I've been tracking my car, I've been using Castrol SRF, a fluid that is remarkably expensive...but only for a brake fluid. It's not particularly expensive in the context of preparing and maintaining a street/track car.

If you're learning to take over the maintenance of and bleed your own brakes, I think the cost of the fluid you've chosen should be a relatively minor concern.

As always, this is just my 2.
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      05-27-2020, 08:28 PM   #6
vexingv
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thanks for all the advice.

i wound up scheduling a service appointment for later this week with a local shop anyway to get an alignment (i also just changed my tensions struts this afternoon.) and after reading Maynard's first reply and out of some caution, i went ahead and scheduled a brake flush as well. this shop charges extra for hydraulic flush of the ABS beyond the usual bleeding process. the estimate isn't cheap ($180 regular bleed + $160 for additional flush), but thought this will just put my mind at ease and the next time i'll be more careful. however, now after reading some newer comments suggesting i don't need to bleed/flush including ABS, and just bleed it again myself, i'm having second thoughts....

Last edited by vexingv; 05-27-2020 at 08:39 PM..
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      05-28-2020, 11:29 AM   #7
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FWIW, I do an ISTAS cycle (at indie shop) pre-post season b/c I swap fluid types (c.$180, including Motul fluid, so not terrible pricing). They told me they got several ounces of clearly bad fluid from each caliper that had been left behind by hand bleeding procedure. And when you are fully opening up the lines (like for SS replacement lines) it helps to depress the brake pedal an inch or so - this closes off a passage in the MC so you won't drain out the entire system (they sell a ratchet-stick tool, but you can jury-rig your own easily).
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      05-28-2020, 12:43 PM   #8
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Quote:
Originally Posted by Maynard View Post
FWIW, I do an ISTAS cycle (at indie shop) pre-post season b/c I swap fluid types (c.$180, including Motul fluid, so not terrible pricing). They told me they got several ounces of clearly bad fluid from each caliper that had been left behind by hand bleeding procedure. And when you are fully opening up the lines (like for SS replacement lines) it helps to depress the brake pedal an inch or so - this closes off a passage in the MC so you won't drain out the entire system (they sell a ratchet-stick tool, but you can jury-rig your own easily).
Good info!! Next time I flush mine I'll find the abs cycle function and use it. I haven't activated abs at all since owning the car (year and a half summer only) so I should be able to see what your shop saw in my clear bleeder bottle
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      05-28-2020, 08:03 PM   #9
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I feel your pain. I had one last wheel to bleed, thought there was plenty in the reservoir. Then I dropped the wrench and couldn't get it back on the bleeder before air got in the system. @#$@#@$&*

Once air is in the system you REALLY need to cycle the ABS to get all air out of the system. Either ISTA or one of the better code readers that can run the process.

I had done a plain pressure bleed of the system twice and still had a touch of mush in the brakes. It took 3 rounds of ISTA bleeding, and 2 full liters of Pentosin LV, to finally get the pedal back to normal performance.

Plan on going through a couple liters of brake fluid when chasing air. You may even want to consider cheap fluid, and then a final replacement pressure bleed/flush with good stuff once the air is finally gone. Then again, the Pentosin LV is ~$15/L at Autozone, so don't go too cheap. LV is WAY cheaper than Motul or SRF.
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