Being at max does not indicate it has no water, or no air in the system, and it is not contaminated. Someone could have just added fluid to it. If they added a different DOT rated fluid and it is not compatible with existing fluid that can also cause issue you indicated. Has it ever been flushed and refilled since you have owned the car? If not have it done ASAP.
If ABS module failure( it is not "good"), was there a light on the dash or did they mention a code? If the module failed did they suggest replacing it? Brakes are very important and if you have any problem with Brakes it puts life at risk, Not just you but everyone on the road with you.
And it is
Brakes Not Breaks or Breacks.
If this happened to my car, it would be either at an ASE(Automotive Service Excellence(
https://www.ase.com/Home.aspx)) certified(in brakes) mechanic or a dealer and would not be driven till the issue was fully diagnosed and certified fixed. I am not sure if there is ASE in the UK but I presume something similar exists.
But as all have seen me type many times your car, your choice , your money & most of all your life!