Interesting:
I had a 1972 1210 4x4 Intl. Travelall with 392 V-8, that I used to pull my boat.
It scared me every time I went to a steep launch ramp with my 6,000 lb. trailer load.
I bought the "Mico Lock" system, and added it to the Travelall.
It locked all 4 brakes, and I never again worried...