1999 Toyota Camry Brakes Locking After Flush

So I'm about to buy this 1999 Toyota Camry, and the guy told me that it used to
have a cracked brake booster, thus, had a hard pedal, and took a lot of force to
stop.  The replaced the brake booster, and bled the brakes.  But then he said
the brakes locked on him, and he had to loosen some hose on the master
cylinder to vent some pressure so they would free up.  Kind of a controlled leak
there, and he said since both sides lock up, not just one side, it's unlikely to be
a brake caliper, and that research tells him that the brake master cylinder is
shot.  Is that likely the case?  Or is there other more likely issues?