What happens if the master cylinder goes empty?

[byez]

During bleeding. Will it just take a few extra pumps to get air out of the system or does a bench bleed have to be performed?

 

[deven]

few extra pumps should get the air out. no need for the bench bleed.

 

[The Critic]

If you have ABS, you might need to use the factory scan tool to cycle the system.

 

[mechtech2]

Since the MC is at the physical top of the hydraulic system, it probably can be refilled and it will be OK. Just step on the pedal a number of times, and maybe tap on the MC to dislodge any air bubbles.
This happens in the shop sometimes, and it always is an easy fix.

 

[byez]

I had to bleed the MC to get a firm brake pedal back.

I now have a slight knock sound from the brakes at low speeds, seems like it makes the knock as soon as I release the brake pedal. Could this be related to the ABS system getting air into it? What else could be causing this?

 

[mechtech2]

The calipers or pads are moving a bit, and when they hit their limit, you hear a knock.

 

[byez]

Is this something that will eventually dissipate or is there a fix?

I tightened up all the calipers and torqued the wheels to spec.