Atwood surge brake replacement

[Rogers200Horizon]

1995 Four Winns 20' single axle trailer with Atwood surge brake with manual reverse lockout. Tried to find the reservoir fill cap, but it seems to be discontinued and out of stock everywhere. Now looking at replacing the coupler, but not sure how to determine if I have free backing brakes or not. It seems like there would be no need for a lockout if I had free backing. I do not have wiring for electrical lockout. Any advice on which direction to go?

 

[dingbat]

You have to pull a drum to look at the mechanism