I have failing brakes on my 1999 ford f150. Sinks all the way to the floor and still will not stop. I do not see any leaks behind the tires or under the truck. How would I know.if i need to change the master cylinder or the abs sysytem?

if no fluid is leaking out anywhere, change the master and pray it's not an abs failure or bad booster

Thanks scotty. Appreciate the quick response