Can an Overfilled Master Cylinder Result in Damage?

Long Story short: Brakes replaced at local dealer (courtesy offer from Toyota Corporate). I paid dealer, submitted invoice and received reimbursement. Service tech replaced front pads and rotors. Everything seemed fine until the day I popped the hood to detail the engine. Noticed that the fluid level in the master cylinder was at the cap and the cap itself had "popped" due to the excess pressure in the system. Brought the car back to the dealer. I was reassured that after draining off the excess fluid, the braking system would be OK. I disagreed. Thoughts/Opinions on this?

 

I insisted that the dealership flush the system as I was worried that the cap, having popped open due to the excessive pressure allowed the fluid within the system to be exposed to air and thus, moisture. Since the dealership (and I) know that it will take a while to get the job done, they have made an appointment for me to bring the car back.

Meanwhile, the fluid level was readjusted. I could have done that myself but I wanted the service manager to see the "quality" of work that was coming out of their shop. I was shocked that the mechanic did not remove the cap before compressing the caliper. Result? There will be a "meeting" between the employees and the service manager to discuss this event. Hope it helps.

 

...Exactly what I told the service manager. I believe that the mechanics are under such pressure to get the job done as fast as possible that they get careless... although I can not understand how or why an experienced mechanic could or would fail to do this. It is my impression that the hood remained closed and that the brake fluid reservoir was neither checked before or after the job was completed.