What causes air to bleed into the master clyinder when pumping the pedel?

Question

Pressing pedel and rear res shoots upi air into the res.

Answer 1

Without getting too technical, the residual check valve in the rear portion of the piston assembly has failed, and will cause air to bleed out into the brake fluid in the reservoir. It will also cause the brakes for the rear portion of the master cylinder to either be ineffective or fail altogether, possibly showing signs of spongy pedal.
Replace the master cylinder, and bleed the brakes.

Happy Motoring