How is lost fluid replaced when bleeding brakes?

Question

(Embarrassing question. I had a vague sense of how the master cylinder (M/C) works; but, when I look at the pictures, I don't see it.)

When bleeding (per schematic below):

Push on the pedal. The piston moves right, and closes off the intake port. Open the wheel cylinder bleeder. The M/C's piston cavity (where the spring is, in the picture) is compressed, forcing fluid out through the bleeder. Close the bleeder. Release the pedal.

The piston wants to retract to the left. The piston cavity needs to expand. For that to happen, new fluid needs to be supplied from the reservoir to that cavity. But, isn't the intake port closed off at this point?

Q: How does the bled-out fluid get replaced in the M/C piston cavity?

Answer 1

Note the shape of the rubber seals in the master cylinder in your diagram. They are designed so that they expand and seal tightly when the pushrod is being pushed but they will collapse slightly and allow fluid to pass them when the pushrod is pulled back.

From your diagram I've added a couple of arrows that show the shape of the seal and how pressure from braking "inflates" the seal causing it to seal more tightly the more pressure is applied:

Then when pressure is released, I've added another diagram with some arrows that shows how the seal can "deflate" if needed to allow fluid to pass into the braking circuit if needed:

From the page that HandyHowie referenced, you can see this image of an actual master cylinder piston and the shape of the seals is clearly seen:

Answer 2

Examine the drawing carefully as its drawn with the brake pedal in home position (up or to the left in the drawing) with both piston chambers open to the reservoir. When pedal is pressed, both feed ports are closed off by both piston seals to allow hydrauilic pressure to build up to force caliper/wheel pistons outwards resulting in disc brake pads/brake shoes against rotors/drums to slow a vehicle down. Holding pedal down, one caliper or wheel cylinder bleed valve is opened to release pressure with air and oil leaving into a container. A good brake system free of air will have the pedal about half way down before opening a bleed valve then suddenly sink to the floor as brake fluid pressure is released thru the bleed valve. Close the bleed valve then pedal is slowly released back to home position to open the two ports to the reservoir. A slight vacuum is created in the one line to the bled caliper/wheel cylinder as the piston travels back to home position, the intake ports open, immediately allowing fresh fluid into the cylinder with the vacuum, filling it. The reservoir level sinks. Repeat the bleeding procedure as the master cylinder ports to each piston automatically refills the chamber that has a slight vacuum. Always top off the master cylinder to prevent emptying it and sucking in air, erasing efforts only to start over again. All hydraulic brake systems are configured to have one piston serve right front/left rear and left front/right rear. If one brake or line fails, ruptures, develops a leak, only half the brake system fails with the other half still able to bring a vehicle to a stop albeit very slowly and taking a longer distance to stop. Half brakes are better than none in a worse case scenario.

You can search for additional tutorials on how brakes work.