Ah, I misunderstood you. Yep, you’re completely right.
Have the rear brakes inspected. Driving/towing a vehicle over a distance with the parking brake on can overheat the rear brake assemblies to the point where it can distort the cups seals in the wheel cylinders causing the wheel cylinders to leak.
Tester
I’m not sure the towing company, while admittedly making a mistake, caused the master cylinder to fail on a 21 year old car; assuming the master cylinder has actually gone bad.
I might question the shop that said the brake shoes were glazed, showed overheating, and still proclaimed they were fine though.
The point could be made that rear brake shoes that are way out of adjustment (possibly aided by being burned off some) can mimic a bad master cylinder. Back to the shop that proclaimed the shoes fine maybe?