Master Cylinder

[Larryjb]

I've been curious about what follows here for some time. Normally when we do a brake job, we bleed the brakes to remove air at the wheels. Now, most of the air gets expelled during the bleeding, and in theory, all the air should be expelled from the system.

Now, if a tiny amount of air may happen to remain in the caliper, perhaps if some one was using the old "pump the brake pedal" technique and a positive pressure isn't maintained as the bleed screw is tightened. Over time this air bubble may rise through the system as it is less dense than the fluid around it. Ultimately, these little air bubbles may come to rest at the master cylinder.

I plan to try bleeding the master cylinder one of these days to see if I find any trapped air here.

Edit: Might be onto something here: https://www.brakebleeder.com/removing-trapped-air-angled-master-cylinder/

 

[afpj]

I'd think an air bubble from a caliper would get stuck at the abs module. Still, good info.

 

[Larryjb]

The hydraulic route does have a continuous path from the master cylinder to each caliper, bypassing the ABS unit. This would allow small air bubbles to travel freely up to the master cyilinder. Remember that when we bleed brakes, air in the ABS pump does not get removed until you actually run the ABS pump.