Since Cranbrook, the thrust reverser systems have been designed to keep the isolation valve open until the reverser has completely stowed and locked, and the actuators are sized to insure they can overcome any aerodynamic forces at high engine power.
td, this is not entirely correct, as the loss of Lauda Air Flight 004 (the first 767 loss) sadly proved. That was the seminal event that resulted in a truly fail-safe thrust reverser design.