Is there a way to set a mechanical brake for non manual modes?
I can control a brake using a mix on my Tx in manual mode and rc pass though but can’t find anything in the config for a brake servo output.
I have a large scale truck with a disc brake and my Tx mix has above mid stick as throttle, below mid stick the first 75% has the motor un powered and controls the brake and the last 25% releases the brake and controls reverse.
I have a parking brake servo on my Rover mower. I’ve controlled it through Lua scripting and also without scripting by OpenTx transmitter mixing.
Do you want to use yours to slow during low throttle in guided/auto modes?
EDIT:
If that’s your intent, I don’t think ArduPilot supports it natively. If you enable the ATC_BRAKE parameter, it may be possible to script brake control by checking for forward velocity with a reverse throttle commanded and applying the brake proportionally. There’s a bit of a safety concern there, depending on your vehicle’s size and characteristics, because Lua scripts are afforded very low priority for processor time.
I want it to use the brake in auto or guided modes, its currently going through conversion from a 50cc petrol engine (its big!) and straight rx setup to electric.
Electrically wise its sorted (just need to fabricate a motor mount), all wiring is done and things like the GPS and lidar are functional and it is working the steering and throttle just fine but my complicated mix on the throttle stick does not seem possible having scoured the parameter list, I can’t find anything for a physical brake, just motor braking 
My mower is 726cc, so size is a bit relative, but suffice it to say that I’m familiar with “large” rover control. Still, both of our projects pale in comparison to a few of the vehicles that ArduPilot has been used to control (big tractors, full sized vehicles…). I’ll do a little research on the subject, as it’s of interest to me as well. Perhaps there’s a way to mix the signals more reliably than scripting.
I did a little more digging on the topic last night and this morning. This is the only similar discussion I found, it’s 5 years old, and I don’t see any follow-on interest.
I’m a bit surprised by the lack of info/discussion, as your case seems a logical one for inclusion in the software, but it doesn’t look like it’s ever been pursued.
I can see three possible solutions, none of which are exactly simple (programming knowledge required):
Fork the ArduPilot source code and write the functionality you need into it (probably in AR_Attitude_Control).
Use a separate microprocessor as an intermediary to interpret the throttle PWM signal and provide the mixed output you require (this link might help with that).
As I mentioned before, use a Lua script to actuate the brake as needed (probably by applying the brake proportional to vehicle:get_control_output(CONTROL_OUTPUT_THROTTLE) and/or ahrs:groundspeed_vector()). This is probably the least desirable method, particularly if braking is critical to your vehicle’s controllability.
So I’m not going mad then, I searched and thought I had to be missing something when I couldn’t find brake options, its bonkers its not part of the config!
Its been confirmed by an ardupilot dev that a mechanical brake is not currently supported, bizarre considering everything ardupilot is capable of but that’s how it is.