Highway driving for a few miles then gradually downshifts so that I need to pull over - Page 2

andrewwynn · #11 10-18-2025, 01:39 PM

#2 is the check valve. To release the vacuum you have to pull that out or release the MC which just pointed out you may have to double up the gasket if you wanted to shim the MC father from the booster to alleviate the issue of the brake booster rod too long.

#2 is just press fit grommet but I'm sure it's pretty tight to keep a seal. It's also under vacuum getting sucked in so there's prob 5-10# extra force just holding it in as well.

Here's a video to understand the concept. Most boosters have an adjustable pin but the modern BMWs i don't think do.

https://youtu.be/vrO7ZZB4dUE?si=I9HaLpcsePy9bOLG

Looked up in the repair manual and there's an o-ring it looks like for the seal and only two bolts.

In addition the e70 has the "pressure rod" aka booster pin attached to the MC not the booster and not adjustable. It's quite possible the adjustment is just off half a millimeter and you can use very little shim to hold the MC a bit away from the booster but not need to use a thicker or doubled o-ring.

The repair manual said to pump the brake pedal a few times to bleed the vacuum. Not sure if the pedal gets too firm to do that but i would try before working on the issue.

If this was my car i would loosen the two nuts on the MC an equal amount say 1 turn and put a 1mm shim between the MC and booster. If that works but pedal has too much travel before the booster kicks in and helps, reduce the shim size and snug the nuts.

The last time i worked on this problem was on a Ford F150. It did exactly like you described. Cracked a brake pipe to release the brakes and took off the MC to adjust the rod length. The weird part in that case was it worked fine for months then wigged out when the ambient temp got hot enough to expand the length of the booster pin/rod.

95% confidence the MC pressure rod being too long is entirely the problem.

Hopefully the o-ring is thick enough to account for shimming out the MC and you don't have to go to the extreme of remove the MC and file down the length of the pressure rod.

–awr–

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e70jane · #12 10-18-2025, 07:53 PM

Ok, so I found time to check out my x5. To my surprise, it drove out of my driveway just fine, so I took it for a spin. I got on the highway going 65-70 mph for like 5 miles, and then the problem started again. I saw the rpm dip slightly and then it started slowing down gradually again. I pulled over to the emergency lane. I shut it off and sat for about a minute and fired it back up. It was ok again so I took the next exit to get back to the streets. I drove about 5 miles to get home driving around 35-45 mph with no issues. Apparently, it seems to happen when I’m driving on the hwy.

andrewwynn · #13 10-18-2025, 10:01 PM

It's most likely temperature that causes the issue; thermal expansion.

–awr–

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e70jane · #14 11-09-2025, 09:19 PM

Quote:

Originally Posted by andrewwynn

It's most likely temperature that causes the issue; thermal expansion.

–awr–

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Andrew, I’ve decided to pull out the new mc to compare it to the old one. It completely validates what you said. Check out the uploaded photo. I’ve decided to reinstall the old mc because honestly, it didn’t feel like it was the mc that went bad. I believe it’s a vacuum problem somewhere, hopefully not the booster, and that it just happened to get disrupted one morning during start up. I remember that it was fine again the very next day. In the past, I’ve driven a couple of cars where the mc started to go bad. It was a different feeling. It always gave a heads up with the pedal having some pressure left before sinking to the ground. Mine was different, as it felt like something just didn’t turn on, causing a vacuum issue. But all in all, I’m so grateful and thankful that you’re around to help people here, and rather than being one of those trolls who feed off of drama that doesn’t even get to the point. I wish I could buy your a 🍻. I’ll be back to let you know how it goes once new fluid is put in, abs bled, as well as the 4 corners. ✌️ brotha.