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

This is what F150 Repair manual says about CHECK BRAKE SYSTEM 

不知你是否自己已经检查过ABS code, 是否要自己维修。如果能自己检查，不妨先看看每个轮胎的ABS speed sensor 及wire是否有损伤，接口是否良好。

另外也有用户反映，更换brake vacuum hose解决了问题。

“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/