Dear sirs,I have a Ford F150 4x4 1998,if I have to do a panic stop, the brakes dont stop inmediately the car,I change the brake pads,but is the same,can you help me? Bernardino

Get the most out of your brakes, be sure to leave enough space at least 3-4 car lengths between you and the car ahead of you allowing you to stop without panicing and slamming on the brakes, causing excessive heat build up etc.(don't tailgate drive).
Also try to replace your old brake shoes/pads with a better quality, don't just look at price, cheaper means less metallic and quicker wear of the pads than the better ones.
Make certain you dont have any brake calipers that have sticky pistons causing the pads to rub excessively on the rotors, not allowing the pads to come back to there rest position.(retract)
Be certain you don't have leaking calipers/wheel cylinders as well and check the brake hoses for cracks, breaks, leaks, replace those that do.
When replacing the disc pads be sure to use all new brake hardware to keep the pads and calipers in good working order.
Use the proper brake fluid the new cars are being called out for today, generally type lll or lV.
Happy Motoring!!!