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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