Did the MC bench bleed okay? No leakage from the seal, all the air came out of it?

 

I had similar issues when I out on new brake lines. I needed a new booster. Research a bit on how to test your booster.

 

Yup. It checked out okay.

I really appreciate all your responses. It's always helpful to bounce idea off of other.

I'll look into it for sure. It's really the only thing we haven't truly tested but from experience it behaves like a car with a huge air pocket or a bad MC. I didn't look into the cost of the booster and it's one of those parts I can't really afford to try.

 

You know better than pumping brakes with bleeder screw open? May be stupid question but that's what you replied with.

My experience with bad boosters are the pedal's good but takes tremendous effort to stop.
......

 

Then you also tried activating the ABS while bleeding?

 

Correct, pumped brakes to build pressure, open the bleeder with the pedal pressed down.

Yes, ran the module bleed about 8 times total. Bled each channel about 11 times total. Absolutely no air came out and ran through about a can of Ate DOT4 in the process.

So far, we've gone through about 4 cans of Ate DOT 4 brake fluid in total. I'm pretty confident there is no air in the system.

What are we missing?

 

Are the master cylinders aftermarket or rebuilt? There is a chance that if they're rebuilt or assembled by the same company, they could have a bad batch of parts. I have had bad ones occasionally, but not usually more than one. That doesn't mean it can't happen, I guess. Aftermarket parts have taken a severe nosedive in recent years. I can't tell you how many BWD parts we get at the shop are bad right out of the box. I think BWD stands for "Broken When Delivered" from the quality of the parts.

If nothing else works and the air is just not going to come out by forcing it down the lines. Empty out most of the fluid in the master cylinder and pump fluid in backwards through the bleeder screws. Air will travel up the lines easier than down. I usually only use this method on motorcycles, but if all else fails, it's worth a try.

 

Both new master cylinders are new made by TRW who I believe is the OEM. We considered sucking the fluid through the bleeders as well but have a hard time accepting that we actually have to. We've never experienced anything like this.

I was a BMW tech for 6 years and currently work as a service manager for a BMW shop. My friend has been a BMW tech for 14 years now and owns his own BMW shop.

 

Yeah, I hear you! Very strange problem!
TRW could very well be OEM. That and the fact they're not a cheap Chinese company, makes me think, it's probably air in the system being difficult!
Brakes are such a simple system, I can't imagine it would be anything else.
I have had some pretty difficult cars in the past. But usually once I get about five minutes in and don't get a good amount of pressure, I'll let it gravity bleed for ten minutes or so. If it still won't build pressure, I open a bleeder wide open and pump the brakes fast until it shoots brake fluid out. Then just bleed them the normal way. I know it sounds counterintuitive, but it works 99% of the time.

 

Please let us know what the problem is when you find it. I am really curious about this one!!