Brake problem.

Hello friend:
To diagnose your Mustang's problem, I suggest that you do three basic tests and they are:
1.- With the engine off the brake pedal and put the engine running if you feel that the brake pedal begins to yield, means that the booster is working well and if the pedal remains hard or has not yielded, check first if the empty hose that connects the multiple of admission and the booster does not broken or plugged, if everything is fine in the hose, the booster relief valve.
2.- Purges the brake hydraulic system, begins with the bolter farther pump (the wheel farthest from the pump) and sees that the pressure that reaches there is adequate. If the pressure is very low, check the dosing valve, it could be dirty or it could have caught an air bubble, it would have to purge it (the largest of dosing valves have an independent vein and if it does not have it purg it by loosening the pipe) verifies if the pressure of the fluid that reaches the valve is high. If the pressure that reaches the dosing valve is very low the brake master pump is evil 8 disassemble, clean it and change the accessories). Something that you should take into account in the purge of brakes (bleeding) if you notice that there is too much air in the liquid that discharges the pothoguer (brake liquid becomes foamy or white is because of air microburbujas) there may be a loose or cracked pipe or a brake earrier that is making a bad contact with the base and could be sucking air through there.
3.- Disassemble the brake drums and step on the brake pedal, verify whether the auxiliary cylinders (brake hypes) act. If they do not act, they could be locked by oxide (clean them and change the Jebes). Verify whether brake tales have good contact with the drum. If there is a bad contact, the balatas change (they do not reevulcanites, there are never couples, it is better to change them for new ones) the drums make them rectify could be oval or very smooth and the tiles slip when braking.
All these failures remove the efficiency to the braking, review them carefully it may be that there is your problem.
I hope I have contributed to the solution.

Greetings. Tarapoto _ Peru