Our truck was doing fine then one day my husband left work and went to stop and when he would press the brakes they will go all the way to the floor and they make a hissing sound the whole time the brakes are being pushed. It's extremely difficult to stop the truck. He checked brake fluids and it's still full and we havent noticed any leaking on the drive way or anything ..please help. At a loss
A hissing sound when the brake pedal is depressed sounds like there could be a leak in the vacuum servo, also known as a vacuum booster or vacuum brake assembly. This is the device that uses suction from your engine to deliver your power brakes, the sound you hear is the suction from the engine's vacuum system being pulled through the servo.
How it works: When you push on the brake pedal you move a diaphragm in the booster, causing it to use engine suction to pull on the diaphragm, making it easier to push the pedal. The booster pushrod in turn pushes on the master cylinder plunger, and it applies the brakes. Your booster always hisses when you push the brake pedal it's just that you don't normally hear it because there's a baffle to reduce the noise, however if there's a rupture in the diaphragm inside the device it can make a lot of noise. You might also notice the engine running rough when the braking, or even stalling due to the loss of vacuum.
However, a failing master cylinder would make your brakes hard to depress, not make the pedal go to the floor without braking action. That problem sounds like you have a failure in your master cylinder. Ruptures in vacuum servos are usually caused by a failing master cylinder letting the brake pedal being pushed too far too often. There are seals and springs inside the master cylinder that wear out, letting the plunger move too far, it usually happens over time so you wouldn't notice anything.
To me it makes sense to replace both components, although it's possible that your servo is still okay I personally wouldn't trust it anymore. They are often sold as one anyway, especially if you are going for a refurbished unit, which is a good way to save some money as long as it's done by a reputable shop.
If it was my car and I'm doing the work I would replace both components, then bleed the brakes on all four wheels, although if you have ABS there might be some extra steps.