Identifying my supplemental braking system?

It's definitely a Roadmaster Brakemaster 9060 system. I've installed 2 of them in our motorhomes. If it's an older system the tank drain valve will be similar to the drain petcock on your radiator . The later models have the ball vale as in the picture you provided.

As for the proportioning valve it's mounted to the frame rail and taps into the hydraulic brake line. As the brake pedal is depressed the hydraulic pressure moves a diaphragm in the valve. This in turn proportionally opens the air valve and allows air to go to the cylinder attached to the brake pedal in the towed vehicle.

Here's a link to the proportioning valve:
https://www.etrailer.com/Accessories-and-Parts/Roadmaster/RM-900001.html

Here's what the Roadmaster towed vehicle air cylinder and mounting bracket look like:
https://www.etrailer.com/Tow-Bar-Braking-Systems/Roadmaster/RM-9060-900002.html

The round bracket mounts to the floor under the driver's seat. The clamp attaches to the brake pedal. I would be hesitant to use the one you fabricated in that I doubt it would have the same rate of response or amount of force applied as the one from Roadmaster that's calibrated for the system. I doubt you'll get proportional braking using a smaller diameter cylinder with a longer stroke. Also keep in mind the system is supplying sufficient air pressure (100 - 120 psi) to operate what's known as a dead brake pedal. I would be surprised if your pedal mounting system (looks like wire ties) could withstand the pressure.

Here's a link to the Roadmaster installation and owners manuals:

Installation:
http://roadmasterinc.com/pdf/85-1811-19.pdf

Owners Manual:
http://roadmasterinc.com/pdf/85-1991-16.pdf

All the manuals and parts you might need are available from Roadmaster:
http://roadmasterinc.com/index.php

This is an excellent braking system. We used it on our 2001 Winnebago Adventurer for well over 100,000 miles without a problem. We left it in the motorhome when we sold it. We have the same system in our 2013 Adventurer. Again it's gone over 26,000 miles without a problem

Be sure to read the owners manual. Part of the towing procedure includes pumping the brake pedal in the towed vehicle to deplete the vacuum in the power brake reservoir before attaching the cylinder. If there is still vacuum in the reservoir the brakes could lockup when the cylinder is activated. There's also a section dealing with "active" braking systems. These are systems that have power assist whether or not the vehicle is running. If you have a vehicle with this system you'll need to add a pressure regulator to the cylinder in the towed vehicle.

Ok. Definetely a Roadmaster Brakemaster system. Thanks all for the replies.

Roadmaster makes a coiled air line that includes the braking-light wire.

Thanks guys. I was scratching my head trying to figure out why the dash LED was labeled "FIT Brakes". Then it occurred to me he probably towed a Honda FIT!
Tomorrow I will call Roadmaster and ask how to check this system. What air pressure should be at that female coupling next to the hitch?
@Ivylog- It has a proportional valve that plumbs into the hydraulic brakes, so it gives air pressure at the same rate. I'm also going under to look where the proportioner is plumbed into the hydraulic brake line.

There's a proportioning valve mounted on the frame rail somewhere or a mechanical valve mounted above the brake pedal.

BTW, that little terminal under the air outlet is for your braking confirmation light.

You can download documentation here: http://roadmasterinc.com/products/braking/brakemaster/brake_m.html

I see now that the larger red hose is nothing but a tank water drain. Must be more to the system as I do not see how you would have proportional braking from the pictures above.
I made my own using a 1.5" air cylinder.

Good idea connecting the tire pressure fill and stepping on the brakes to check the output pressure of the system. Also for leaks etc.. Here is a picture off of Roadmaster's website of their compressor. Obviously it is the same as mine. I was just looking for someone who has the same system in their motorhome. I guess I'm just looking for reassurance before I spend the money to outfit my Saturn with the rest of the system.

Even if you find who made it, you still have to test it to see if it still works. I would rig a sliding tire pressure gauge to the output at the box and go out and put the brakes on hard. Hopefully you will have recorded a psi of 50+. I made my own air braking system using a 1.5" diameter air cylinder that fastens to the toad brake pedal.

If it is Roadmaster, it is an old one. It could also be an M&G system. They did make a system for gas coaches and it had a big box with a pump in it.