“Displayed when a fault has been detected by the ABS module”

This is what F150 Repair manual says about CHECK BRAKE SYSTEM 

不知你是否自己已經檢查過ABS code, 是否要自己維修。如果能自己檢查，不妨先看看每個輪胎的ABS speed sensor 及wire是否有損傷，接口是否良好。

另外也有用戶反映，更換brake vacuum hose解決了問題。

“I have a 2011 F-150 XLT that was giving me a "Check Brake System" warning only after driving on open highway for a short time. I checked everything I could prior to going to the local dealership. They could not duplicate the problem. Another dealer told me to have them check the vacuum sensor on the brake booster. They changed out that and the vacuum hose and I've had no problems since then. 
Good Luck!!!”

https://www.f150forum.com/f38/check-brake-system-error-message-dash-screen-329142/index2/