Just Google the lucas girling brake system.
Save a lot of typing.

And yes the park brake can apply itself while driving if you were to rupture a Hyd park brake line.
You loose pressure and the spring inside takes over and set the brake.

Tom

1951 GMC 650 Diesel
1958 GMC 550 Wrecker
1964 Chev C-80 427cid Tandem
1947 Dodge WDX Power Wagon
1977 Chev K-30 400 cid
1984 Chev K-30 Cummins 6BTA