No brake pressure on front circuit after master cylinder replacement
 

2007 BMW X5 E70 4.8i.


Original problem was a hard brake pedal despite good vacuum at booster so I replaced the booster, which involves removing and reinstalling the master cylinder then bleeding the system (including DSC bleed via ISTA). Brakes had boost again, but pedal would slowly sink to the floor. I suspected that when I bench-bled the master cylinder it may have stressed the old seals, so I admonished myself for not replacing the master cylinder the first time and went ahead and installed a new master cylinder.

After thoroughly pressure bleeding, DSC bleeding via ISTA, and 2-person manual bleeding the pedal will sink almost to the floor with minimal resistance, as if there were a problem with the (just replaced) master cylinder. There are no bubbles in the system and no brake fluid leaks anywhere. When I pressure bleed I get clear fluid at all 4 corners so the circuits aren't blocked. When I 2-person manual bleed, no fluid comes out of the front brakes.

Since the front brake circuit is not blocked (pressure bleed works fine), the problem must be at the master cylinder which is not supplying pressure to the front brake circuit. Any ideas? Could there be air in the front circuit portion of the master cylinder? Bad master cylinder? To be clear, I bench bled the cylinder before installing it (even though the BMW TIS doesn't require it), and additionally I slowly cycled the brakes while pressure and DSC bleeding as per the TIS.

If pedal sinks with no link the only place i can think of is the piston seal for the front circuit.

I've noticed my pedal will very slowly sink under constant pressure so I'm due for MC rebuild.

New ≠ fixed, you have to follow the symptoms even if it means the brand new part is the likely candidate.

It's very strange to have zero pressure.

If you hit the pedal hard but don't hold to you get resistance? (Suggests weak vs. missing o-ring).

If the pipes are long enough you can confirm MC fault by swapping front and rear output and the problem will go to the rear.

(I'm not sure there's not an additional bypass path with the abs pump and block)

Nope - pedal sinks about 90% to the floor no matter if I press slowly or hard.


Come to think of it, I recall hearing a "pop" noise right after I installed the master cylinder and initially pressed the brakes. I chalked it up to the plunger seating in the piston, but now I wonder if the front circuit seal gave way.


There's no room to swap front and back circuits at the master cylinder - the lines have almost no give, which makes installing the master cylinder a real chore.


Is there any way to bench test the MC? Perhaps plug the lines and see if the piston can be depressed?

That'll work as well as swapping lines.

Plug the front outlet. Press pedal. If it firms up, not MC.

The plungers are usually attached via a mini teeter totter and maybe one side pin broke?

Removed the MC and bench tested it. Sure enough, the front circuit had failed completely and would not build any pressure at all.


FWIW it was a new Beck Arnley from RockAuto with a 1 year warranty. Let's see how the return process goes...

This isn't really making sense, what was your bench bleeding procedure ? Do you have a kit with hoses to return to the reservoir? By "no pressure" you mean no air either? You were not able to "pump up" the brakes at all when bleeding?

Is the front of the master cylinder also the front brakes ?

Yes, I have that kit. When bench bleeding both circuits were pumping, so I suspect the seal gave way after it was installed on the vehicle. That could have been the "pop" I heard.

No air either. Not able to pump up the brakes at all - pedal would sink 90% of the way to the floor at which point the rear circuit was pressurized by physical contact of the primary piston (look up how dual circuit master cylinders work when the primary circuit fails).

The front circuit is the primary circuit which is closest to the firewall.


New MC is on the way. I will not be bench bleeding this time just in case my bench bleeding procedure which pushes the piston all the way in (perhaps past normal pedal travel?) risks damaging the seals.

Well guys, I'm losing my effing mind.

Removed the (new, recently-replaced) master cylinder. The front circuit seal had indeed failed completely - it would not build any pressure on the bench. Sent it back for a refund.

Got a replacement master cylinder. It built pressure on both circuits on the bench while bench bleeding, and I was very careful to not over depress the piston in case this could damage a seal. OK, so far so good. Installed it, thoroughly bled the system (pressure, DSC, and manual). Pedal feels firm and holds when vehicle is not running. When the vehicle is running (brake booster on) with firm pedal pressure the pedal will slowly sink 80% to the floor which is the same symptom I had previously where the front circuit seal had failed. Pumping the brakes firms things up, but the pedal will sink to 80% if you continue to press hard. I suspect that this 2nd new master cylinder has a leaking but not outright failed front seal.

Again, no bubbles, no leaks.

Unless someone has any insight, I'm going to order an OE MC from FCP Euro. If that works then I'm going to disassemble the Beck/Arnley MC from RockAuto to see what is wrong with the seal.

Maybe the ABS is leaking fluid by ? Do you have a plug you could use to block off one circuit and see if the pedal still drops ?

I don't understand how the ABS/DSC could leak fluid internally without the fluid level in the reservoir dropping. Where would the fluid go?

Not off hand. I guess I could make one up with a bolt with the correct thread pitch and teflon tape, but I don't have anything to seat on the flare. This is a good idea, as it would confirm or refute my suspicion. I'll see what I can cook up.

If the ABS is plumbed like it was back in the day then it shouldn't happen but who knows now. It seems pretty strange to have 2 replacement master cylinders leaking by. The best way to make a plug would be with a piece of brake line. That would for sure tell the story.

If there is a proportioning valve that could be bottomed out (maybe the clunk you heard) just something more that might be going on.

Maybe not "leaking by" but retracting ?? Have you tried driving it ?

I found an M12x1.0 double flare plug online. I'll see if I can get one locally.

Don't understand what you mean by retracting. If everything is working properly you should get pressure when you press the pedal, and the pads should retract when you release the pedal. Haven't driven it because it's not working as expected and it's time consuming to put everything back together, put the wheels back on, and take it off the jack stands only to have to get it back up on jack stands again.

We've had some people who have had problems with a sinking pedal and ended up needing to drive the car to get the ABS working but I think sometimes that didn't even totally seem to fix it.

Replaced MC with OE (TRW). Brakes feel and function fine now.

Was difficult to bleed the system. I bench bled the MC before installing it, then:

1. pressure bled
2. pressure bled + cycled DSC
3. manual 2-person bled
4. manual 2-person bled + cycled DSC
5. manual 2-person bled with pressure bottle hooked up
6. repeated above steps

Front circuit was really hard to bleed. Brakes were feeling awful until step 5. For some reason that blasted out a huge amount of air that no step before it did. This leads me to suspect that even though I bench bled the MC the act of installing it reintroduced air into the MC. Scotty Kilmer on YT advocates "burping" the MC by applying brake pressure and briefly cracking open the line at the MC which I guess provides any air the shortest route to escape. I suspect any air at the highest point in the system may tend to remain there despite bleeding efforts.

So first new MC had clearly failed, 2nd new MC was probably OK but the system wasn't bled properly, and 3rd new OE MC worked when system was thoroughly bled. I disassembled the 2nd new MC and couldn't find anything wrong with the front circuit seal. Oh well.

What a nightmare.

Bench bleeding is meant to make the job a bit quicker so you don't have to wait for the master to fill through the compensating port. Cracking the line at the master is normal procedure and sometimes you can get away with not even bleeding at the wheels but more so on older cars. I think what happens here is that getting a bit of air into the ABS is causing the pedal to drop in a way that doesn't really feel like it's air but something is bleeding back/around (maybe piston retracting whether it be unwinding or maybe through an orifice) through the ABS and maybe more so if there is a bit of air and more noticed when a person is focused on working on the brakes! ;)

There are some places like the front brake lines just before they got into the hoses where there are going to be high spots and some pretty good flow (high pressure on the pedal) is needed to get the air through which is why I always recommend using 2 people especially for problem cars.

I replaced the master cylinder on my 4.8 x5 and now I’ve got a major problem. I think the system isn’t bled properly. It was kinda of a sloppy job and I used Mercedes dot 4 lv fluid. The pedal feels ok though so I assumed all was fine. But there’s a problem when I take it on the highway after like 5 miles of driving. The car will suddenly slow down on its own, until eventually coming to a halt, quite dangerous if you’re not quick to act by pulling over onto the shoulder. The car doesn't die out or anything, but when applying gas, it doesn’t want to move. It feels like the brakes are being applied while trying to accelerate. The screen will also show dsc 4x4 warning. When I shut it off and back on again, the screen clears and the car drives normally again. When I take the streets, all seems fine. It’s only when I get on the hwy and drive a for a few miles going 65-70mph, the problem will come back again. I also notice when it acts up, there’s a slight dip on the rpm’s, and then it starts to gradually slow down even when I apply gas. It’s like a gradual resistance that becomes stronger and stronger until it finally comes to stop. I’m actually gonna work on it tmr morning. I will bench bleed the mc while still hooked up to the booster. When finished, I will use a stick to hold the pedal to the floor before connecting the 2 hard lines back to the mc. Next, I will do the 2 man bleed job on all 4 corners to get out as much fluid as possible and keep filling with new valvoline d4 synthetic instead of Benz fluid. I will also use foxwell to activate the abs each time I bleed a wheel. Hopefully, this will work.

That doesn't sound like a problem with air in the system which would require further bleeding. Definitely pull up codes, as the DSC warning and symptoms suggest that the DSC is being applied to one or more wheels when it's not supposed to be.

Do you think it could be the transfer case?

Nope zero chance. It's for sure the boost acting on its own. Checks all the boxes and exactly matches the symptoms.

You should be able to loosen the nuts on the master cylinder to confirm.

It looks like the o-ring seal might be fat enough to perform this test and get the brakes to work properly.


–awr–

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The fact that the problem clears on stop/restart makes it seem more like a limp mode problem then a mechanical issue with brakes. I can't make a case that stop/restart would somehow relieve trapped brake hyd pressure. Are there any other codes?

My '08 3.0si would go into limp mode displaying 4x4, DSC, and other messages, engine revs would jump as trans downshifted. Stop-restart would make everything normal until the next event. My problem was an intermittent crank PS. I found the code w/ISTA. There are many possible causes for intermittent, near complete power loss.

I bled the caliper and abs again, but I haven’t yet driven on the hwy to test if the problem was still there. I did want to point out a strange problem when bleeding the calipers. While bleeding the last caliper, the front driver, loosening the bleed screw and applying the brake pedal, no fluid came out. The pedal was stiff. I had to continuously apply pressure on the pedal to finally see some fluid come out. But even then, it was dripping rather than spraying out. I wonder if this is normal?

Andrew, when you ask me to loosen the mc from the booster to see if the brakes will release, do you mean that once the car starts acting up again, and I am pulled over onto the shoulder, keep the car on and pop open the hood to loosen the bolts?

Yes, it seems like a limp mode condition. However, I never thought limp mode would stop the car from moving. From my experience, it still allows you to drive, but just that power is reduced.

Some malfunction with auto hold could cause similar symptoms if the P brakes applied themselves but if they stopped you from freeway speed you would smell them.

Regarding the master cylinder nuts: if you get the locked up situation, you should be able to loosen them a little bit and it should relieve the pressure.

read some of the threads talking about the push rod being too long and adjusting it. It is exactly the symptoms you describe.

since the problem presented after replacing the master cylinder, it’s probably just a tolerance of manufacture issue.

you can confirm the booster is problem by disconnecting the vacuum line of course plug it with a screw or something and you just don’t have power brakes. You have to push really hard to stop the car, but the problem should go away.

there was a recent thread on XO where somebody had that problem and confirmed it that method


–awr–

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This is not normal. I had a caliper flex hose fail internally that caused this condition. You can loosen MC end of the flex hose and see if flow improves.

It's rare but totally can happen that the brake hose collapses internally.
Back in the day bleed screws could get rusty and cause problems and often we had to take then right out to clean them up to get enough flow to bleed the brakes properly but those days we didn't have rubber plugs on them! ;)

It should be fairly obvious if it's the brakes causing the vehicle to slow down ??

My thought was related. I would have removed the bleed screw to make sure it wasn't plugged/gummed up


–awr–

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I did remove the bleed screw and it was dripping rather than spraying out when I applied the brakes. I wonder if this could also be a potential cause for the problem I am experiencing on the hwy.

Do you have an infrared thermometer?

Weird flow at the caliper might lead to stuck caliper but if just one you will feel it in the wheel it will pull to the side that sticks but also that will get very hot!

You should try driving with the booster disconnected obviously with every needed precaution it will take a lot of force to engage the brakes but it should stop locking up.

Or: loosen the two nuts holding the MC and that should confirm if the booster rod is the issue.

The front left caliper I would consider that part B.

Have you done any of the diagnostic steps to test the booster yet?


–awr–

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You can wait until it acts up or you can do it first to see if you can get it to act up.

The theory is when hot the push rod in the MC is expanding and engaging the booster. The tolerance is very tight like 24/1000 and if they start out touching you will get the exact behavior you are experiencing.

If that fixes the problem the "proper fix" would be to remove the MC, measure the depth into the booster and the length of the rod coming out of the MC and file down the rod until it's the proper length.

Most cars have a threaded rod coming from the booster and and you use a special calibration tool to measure and set it. BMW doesn't they are made one size at the factory.

If this happened to my car i would loosen the nuts to get the rod gap to work by trial and error and then I'd shim the MC away from the booster using shims in numerous places around the base of the MC.


–awr–

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I haven’t had time to test the booster yet. I’ll do it this weekend. By the way, here’s the old mc. If I recall, it looks identical to the new one. I don’t understand what do mean by the extended push rod? Could it have been slightly longer, but I just didn’t notice it?

https://i.postimg.cc/68vmmtHv/IMG-1289.jpg

https://i.postimg.cc/vcjKhcvb/IMG-1290.jpg

Yes you would not notice the difference in length unless you used a digital caliper.


–awr–

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@E70Jane, What happened to the brake fluid being restricted to front driver's side ??

It might be less confusing if you stick to your original thread.

I've never seem a BMW with a adjustable MC push rod. Some have a rod length adjustment on the brake pedal side of the booster.
Need to be careful that shimming MC doesn't create a vacuum leak.
When the problem occurs, have you tried lifting the brake pedal with your foot to see if brake pressure is released?
Is the brake pedal return spring disconnected?
Is something interfering with the brake pedal returning fully?

I think this another one of those threads getting posted in multiple places/forums and everyone is getting a different piece of the story.

Plus probably some private messaging and Andrew Wynn adding to the confusion.

No brake pressure on front circuit after master cylinder replacement
 

Correct.

Regarding [mention]e70jane [/mention]topic of the automatic brakes go here:

https://r.tapatalk.com/shareLink/top...ink_source=app


–awr–

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Thx Andrew, I’ll go back to my original thread. Didn’t mean to hijack this one during my research.

It happens all the time when people just reply to the thread last posted. I was just as guilty not checking the thread title.


–awr–

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