I'm in the process of bleeding my ABS using a scan tool. I also have a new master cylinder which was properly bench bled before I installed it. The scan tool is correctly commanding the ABS to bleed for 4 second intervals and I'm using the front passenger caliper to bleed per instructions. I was getting continuous air in the brake line, after bleeding for over an hour and probably through more than a gallon of fluid. My silicone bleeder hose is super tight on the bleeder screw and I have no leaks or reason to believe I am pulling in air there. When the ABS pump runs, it almost makes a screeching noise and you can literally hear the bubbles crackling from the ABS unit as it pumps. When I remove the cap each time to refill the MC, there is a tight suction on the cap. I can hear the suction release and feel it when I remove the cap.

During one of my refills, my MC cap fell down into the engine bay. I continued without the cap and after just a few more ABS bleeds, the pump was quiet and the air was gone. I don't think this was a coincidence. I assumed an airtight cap was good (my old one didn't do that) but now I'm not so sure. Is the cap vented? Is the suction seal not how it should be? How does the MC replace the bled fluid with air? It seems like the airtight cap is related to the ABS pulling air in somehow.

I hope to update my other thread soon as to the cause of my low brake pedal.

 

Cap is vented (or should be) but it can't allow air to pass quickly, which is not a problem under normal driving since the fluid level (pad wear) drops very slowly.