Brake Pedal Slowly Sinking When Braking

[workingonit]

Originally Posted by Peezen

• 1) I have been watching videos for the e39 and feel that I need to do a better job bench bleeding and installing
• 2) I have the BMW specific Foxwell, I have ISTA, and I have INPA. I should be able to get the right bleeding procedure down
• 3) I have never had this much of an issue with brakes before ....

My points of agreement:

• 1) I've been trained at a GM tech school in '71-72, and done dozens of brake systems over the following decades, on my vehicles and others (I never worked as a pro auto mechanic, but went in another direction, so I never had my own "shop"), but I know that I bled the MC and brake lines as good as I'd ever done before
• 2) I bought my Foxwell with GM software installed a few months prior to getting the X5 (it was still being worked on by the PO, before we traded cars...my Chevelle for the X5 he bought specifically for the trade), added the BMW software to it, and have used the NT510 Elite many, many times on the X5 and all four of my GM cars since then, so I should've been able to get the right bleeding procedure down on the Cobalt
• 3) I too, never have had a soft pedal problem I couldn't resolve, before the Cobalt

So far, I've had no brake problems with the X5, but I would assume that the Foxwell autobleed procedures found in both GM and BMW software has been satisfactorily successful for others prior, and perhaps both you and I were just unlucky when we were trying to bleed our respective brakes.

[80stech]

Yeh, that doesn't sound right, maybe try a different shop. "Machine bleeding," whatever that means, is not the answer. Maybe find a shop with some older guys who know how to bleed brakes and will know what is going on if there is more to it.

[80stech]

Another thing to check is that there is nothing keeping the master piston from going all the way back, shop should know that though.

[80stech]

I think some have said that doing some ABS panic stops has been the trick ??

[nick325xit 5spd]

Originally Posted by 80stech

I think some have said that doing some ABS panic stops has been the trick ??

For me, I generally like to run the INPA bleed procedure a couple times, followed by a traditional "two man" bleed. Note this burns a LOT of power. A 10A power supply is often not enough to get through it.

Triggering the ABS isn't a bad strategy, but it's hard to do that extensively enough to get things totally clear.

Fun fact: Windows 11 ARM x86 emulation works so well that I can actually run INPA in a VM on an Apple Silicon Mac.

[workingonit]

Originally Posted by nick325xit 5spd

...Fun fact: Windows 11 ARM x86 emulation works so well that I can actually run INPA in a VM on an Apple Silicon Mac.

My Arm64 processor laptop isn't nearly as powerful as even a VM application. It has a total 4gb memory (most of which ...usually 60+%...is eaten up by Windows Defender), so I can hardly run INPA, can't get ISTA+ to load, and it won't even download the Windows 11 Microsoft Android Emulator (which I wanted to use for my Android automotive apps.

I'm sure you paid 5-10x for your Mac what I did for my laptop, which I sorta regret buying now, but you've got functionality! As Joker said: "Where does he get those wonderful toys?"; I feel the same about my budget devices.

[Attacking Mid]

To follow up on this, I spent countless hours bleeding my brakes to no avail. I replaced all the rubber hoses that connect to the calipers (they were showing cracks anyway), I replaced the master cylinder, and I followed the BMW bleed procedure to a "T" several times. If anything, I made it worse. I also observed some weird phenomenons with just getting endless bubbles. More searching forums revealed others who have fought this issue to no avail. A couple people mentioned they had success by wrapping their bleeders with teflon tape. Since one of the weird things that was happening was... after shutting the bleeder, bubbles would continue to flow through the clear hose attached to the bleeder nipple, I considered this. That was a head scratcher that indicated air was somehow getting in the line even with the bleeder shut.

I had little hope, but back up on jackstands she went today. All the wheels removed AGAIN. I started by removing each bleeder nipple and carefully wrapping about 3" of teflon tape around each. I then reinstalled the nipples and ran the BMW bleed procedure as I had before. I have a homemade pressure bleeder, and I ran the bleed procedure in ISTA+ for each wheel separately with the bleeder open. I didn't seem to get many, if any, bubbles out of the rear brakes, but the RF brake had a lot of bubbles. I ran the ISTA procedure a couple of times with 5 brake pedal full pumps following each time. Eventually the bubbles stopped, and I moved on to the LF where I got fewer, but still significant bubbles. I then shut the bleeder and put my foot to the pedal and GLORY BE!!!! we had a firm pedal!

Ultimately, the problem was all caused by air leaking around the threads of the bleeders. Intuitively, I seriously doubted that could be my issue, but I was proven wrong for the second time in a decade (sorry, corny dad joke). This problem was really grating on me, as it just shouldn't be this hard to bleed a brake system and get a firm pedal. Anyone need a perfectly good, used master cylinder? ;-)

AM.

[andrewwynn]

Great follow up it will for sure help somebody


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