One problem related to power assist brake has been reported for the 1997 Ford Explorer. The most recently reported issues are listed below.
Lost power assist to braking system during freeway rush hour. No warning of loss of power brakes. Almost unable to stop in time. Lost of power assist traced to loss of motor vacuum due to air filter clogged with auto transmission fluid. Filter recieved auto trans oil from transmission fluid leaking from oil cooler. See service bulletin number 99-4-6, bulletin sequence number 477, date 9903, NHTSA item number sb60476. Dealer says service bulletin does not indicate that air filter should be checked when repairing leak or that a clogged air filter can lead to loss of power assist to breaks. Also note that the antilock breaking system will not work without the power assist.