*Hydraulic surge brakes seem like the modern
choice (versus pure mechanical).

If I understand correctly, the most frequently encountered reason for
using a mechanical surge brake is the parking brake feature, which is
so convenient on our glider trailers. A mechanical parking brake can
be left engaged indefinitely without harm or loss of effectiveness (at
least until the brake shoes attach themselves to the drums through
rusting/corrosion of the drums or actuating hardware which, from
experience, can occur in a relatively short period of time if it's
damp). There are some hydraulic parking brake systems that trap the
pressure in the system and leave the brakes engaged but I don't know
how well they work [can anyone comment?]. As a result, it's relatively
easy to replace the German mechanical surge brake and axle on Komet/
Cobra trailers with a hydraulic version that brakes the trailer and
provides a breakaway emergency feature. But it's far more difficult to
provide the parking brake feature. Without knowing the relative cost,
and having had to make this decision before, I'd generally recommend
rebuilding/replacing components of the existing system if you want to
keep a surge/parking brake.

Chip Bearden
ASW 24 "JB"
USA