Don’t know how I missed this thread while searching for Siskin references.

As surmised previously, it controls the brake on the Vickers ( not Rotherham WW1 afficianados ) fuel system, wind driven, air pressure pump. The fuel tanks are pressurized at approx 3 psi, pushing the fuel to the carburetor. While there are pressure relief valves in the system, braking the fuel pump was required at high speed, most notably when diving.

Mike, I have sent you a PM.

Regards
John