Use the reverse light micro switch in your shifter to trigger a relay

You can isolate the two circuits by using relays to trigger your transbrake instead of feeding your trans brake directly from the delay box - use the delay box to trigger the 2nd relay

The Bosch/TYCO - Relay, SPST 12Volt/30AMP with diode works good for this purpose

By doing this both circuits are not connected and work independently of each other