hello need to get hold of one of these i think its a pressure switch as my brakes keep failing any one else had this problem

i can be driving fine then all of a sudden my pedel go rock hard then no brakes, take the car home start it the next day then its fine again, could be driving 4 ages there fine or five mins then they fail so i dont think its caused by heat

 

I don't know what that bit is but it would be worth having a close look at the pump connector, as you are losing assistance.
tabetha