Can this damage the membrane though? Should the cap get replaced after an overflow?The cap is not air tight. It allows air in so that the membrane can get sucked in as the brake fluid drops with pad wear. Since the brake fluid flowed in reverse, it leaked past the membrane as it is not designed to keep hydraulic pressure.
I only top up brake fluid when I put new pads on on my own car
I usually do that too, but this time I had changed the brake hoses with braided ones and I changed the fluid along with them.