Triple Valve/Distributor valve
Triple Valve/Distributor valve - https://www.google.de/search?newwindow=1&hl=de&q= distributo...
Bilder zu distributor and brake disc - https://www.google.de/search?newwindow=1&hl=de&q=distributor...
A distributor (or "triple valve" as it was always called and sometimes still is) on each vehicle monitors the pressure in the brake pipe. When brake pipe pressure falls, the distributor allows air from an auxiliary reservoir on the vehicle to pass to the brake cylinders to apply the brake. When brake pipe pressure rises, the distributor releases the air from the brake cylinder and recharges the auxiliary reservoir for the next application. - http://www.railway-technical.com/brake1.shtml
The triple valve is now usually replaced by a distributor - https://www.google.de/search?newwindow=1&hl=de&q= distributo...

This is supposed to prevent rear wheels (in this case of the trailer-buggy) locking up sooner than the front wheels, which could cause the bicycle to skid.

Other than that, the more literal translation could be used: "traction distributor". However, I haven't found a precedent for the latter.

It would be tempting to use automotive terms (for trailers), but the mechanism here is a wire one without the use of hydraulics.