Either way will work. Basically, as long as the ABS system is not active, such as when you have the engine off and are just bleeding brakes, the secondary (ABS) circuit inside the ABS valve body is isolated from the primary circuit and it is just the same as bleeding a non-ABS vehicle. So 2-man or ole gravity bleed...take your pick. The fluid just goes straight through the ABS valve block during bleeding. There will remain a tiny bit of old brake fluid trapped inside the ABS unit secondary circuit until the first time it activates and mixes with the new fluid in the primary circuit- no big deal. At least you are getting mostly new fluid in the system!
I have a customer with a low mileage 95 840i, and we had never done any brake work on it since I started working on his cars a few years ago. He was out driving one day sort of pushing things a bit from traffic light to traffic light, trying to get to Fed-Ex before they closed to pick up his new phone, when he completely lost his brakes and nearly rear-ended another car. He called me all upset and asked me if he was going to need a master cyl or something. I asked him when he had flushed the brake fluid last and he said it never been done. Well, I told him to let things cool down and his brake pedal pressure eventually came back and he drove it home carefully. I guess it had so much moisture in the fluid that the boiling point had been reduced a lot. We flushed the brake system soon after when he brought it in for an oil change.
He had previously questioned a few weeks before when we did pads and rotors on his 98 325i, when I said that the brake system needed a complete flush to renew the fluid. He told me he did not know that was ever needed and had never had anyone mention that before. I guess he forgot that we had also bled the fluid on the 98 740i a couple years ago when all the brakes were replaced. After his close call, he had no doubt that the fluid should be replaced at least every time the brake pads are changed.
