Service brake assist message?

When you say, "does not have any codes" did you do a CAN scan for the ABS codes or just a normal drivetrain scan? Here is a video to show you how:

https://youtu.be/u-4syLc-ifQ

Did you check the brake fluid level? I have seen the wheel bearing hub start to go out and cause the reluctor ring to misalign with the wheel speed sensor. Do you hear a slight rumble from the front or rear while driving? Also, with the engine running remove the vacuum line from the booster to see if vacuum is present.

Used an Autel scanner no abs codes. Brake fluid is right under the line. No noise from bearings. Has vacuum at pump and at valve to booster. Had these history codes C0800:12 and U0415:00 and U0121:00 cleared them and the first and third code came back as history codes again. Used Autel to to check abs signals from bearings and had it on graph mode drove it for a while did not see any difference between the four all were reading the same and could not get the message to come back on last night. Also checked the amg battery it is rated 700cca but only putting out 400cca otc battery tester said it failed going to install a new one tonight. Also has new front and rear brake pads installed in September. And replaced vacuum pump like I said earlier.

I have seen bad ABS control module cause these codes,

C0800: Control Module Power Circuit

U0121: Lost Communication with Anti-Lock Braking System (ABS) Module

Here is the instructions on how to replace the ABS controller. Check out the images (below). Let us know what you find.