I presume it is leaking. If not, don't even consider it as if anything, stop leak will make your hard spots harder. An alternative strategy if not leaking too bad is to bring extra
ATF for refill on your trip. If it leaks a little so what. One possibility is if your car has a power steering reservoir filter is it is clogged...not necessarily the steering box contributing to your intermittent hard steering.
HTH,
George