On occasion front brakes staying engaged while driving severe. vibration in steering. when this is not occuring there is hissing noise at brake pedal which goes away when pressed.

Why Does My Car Wobble When I Brake?

www.popularmechanics.com/cars/.../why-does-my-car-wobble-when-i-b...

If your brake pads are still in fine shape but the car trembles when you breaknonetheless, this could be your problem. ... Lately when I hit the brakes, the car trembles. ... In my experience if the steering wheel shakes or the front suspensionshudders when on the brakes, the front ...

Brake Vibration: Common Questions Answered - EBC Brakes

ebcbrakes.com/articles/brake-vibration-common-questions-answered/

Brake vibration is where a shaking motion occurs when the brakes in a car or othervehicle are deployed. This can vary from a slight shaking to a quite severe ..

• Knowing the warning signs of impending trouble can go a long way toward keeping your car from a highway breakdown.
  
  You need to develop an awareness of how your car should sound. For instance, when you turn the key to start, the gear like sound of the starter motor engaging should be smooth and sound like an electric motor working hard to crank the engine. If you notice a different sound - more like a grinding - chances are you have a starter motor problem about to happen. Let's say you're driving along and you feel the steering wheel vibrating at certain speeds. The onset of this subtle vibration could tip you off to a tire that needs to be balanced, worn steering linkage or a bulge in a tire that may be about to blow out. In any case, the vibration needs attention. Normally you shouldn't notice the sound of the exhaust system. A sudden change in your car's sound, therefore, isn't something to ignore.
  
  Suppose you're used to a pleasant hum, then, perhaps after a bump or when starting up the car one morning, there's a loud roaring that sound like a hot rod without a muffler. The harder you press the gas pedal. The louder the noise becomes. A noisy exhaust means the exhaust gases are blowing out under the car rather than at the rear of the car - a dangerous condition. Brakes play their own warning tune. On many cars, a scratching noise coming from the wheels that stops when you stop pressing on the brake pedal is caused by a low-brake warning device. Its purpose is to warn you that you need front brake pads. A loud grinding noise when you step on the brake pedal is an alarm that your brakes need immediate attention. The grinding noise occurs when the brake pads are totally worn away and the metal of the brake pad backing plate rubs directly on the brake rotor.
  
  Ignoring grinding brakes can be a costly - or even deadly - error. Shock absorbers help keep the car steady and balanced while the wheels roll over bumps and ruts. Their modern counterpart, the MacPherson strut, does essentially the same thing. You might not notice worn shocks or struts when you're simply riding around town. The safety effect of these devices comes into play at higher speeds. When you hit a bump, the tire wants to bounce like a basketball. Worn shocks or struts allow the ties to bounce out of control making steering difficult and braking less effective.
  
  What you may notice is the car rocking more every time you stop, assuming an almost boat like quality. Old, broken or worn out shocks and struts should be replaced to ensure safe steering and braking at higher speeds. Engine noises are easiest to hear when the car is not moving. Transmission sounds or brake noises usually occur when the car is moving. Pay attention to any new sound and keep notes - how it sounds, when it makes the sound and what difference, if any, occurs in driving when the noise is present - to help you describe it to your service dealer.

Look for loose steering components.
, also check Front brake rotor run out.

yes but up to a point
the brake pedal should move about 1" to take up the pedal free play and then be applied smoothly as you press further
the harder you press the more solid stopping action you will get
that way you can feel the brakes working so that you can adjust the braking action for stopping
there should be no hissing noise that occurs when the brakes are being applied
you say hydro booster which is hydraulic booster but a hissing noise will come from an air leak in a vacuum brake booster which indicates that it needs repalcing

if you feel the pulsation in brake pedal when stopping. its a warped brake rotor thats causing it. for bad cv joint symtoms--- Unusual noises when making turns. As you are driving on the road and you make a turn at the corner, if you start hearing unusual sounds coming from under the car then this is one of the symptoms of a bad CV joint. These noises will sound like something is clicking beneath the car every time you turn the steering wheel. Evidence of a damaged boot. Steering wheel feels heavier. The steering wheel will start to feel tighter and heavier and you can hardly make simple turns to the left or right. In severe cases you might even feel heavy vibrations like you are driving down a rocky road.

If you get a bad vibration in your steering wheel at low speeds without braking it is either a bad tire or a bad rim. If you get a vibration at speeds around 45-50+ and goes away you probably have a balance issue. If the vibration only occurs while braking then your rotors are warped. If low speeds it shakes i'd take it to a shop to check your tires.

if it were the rotors it only happens when braking take in have them turn rotors see what happens good luck

remove the master cylinder and check the plunger projection make shore it is with in specs, using a vacuum pump check condition of capsule see if it holds the preasure and its not leaking off. if its leaking change capsule and check valve

Sounds like the brake pads are sticking or the calipers. i would have them checked and have the calipers greased.

Your car has front disc brakes, the disc's are running out of true, as u apply the brake this causes the steering wheel to shake, the higher the speed the more u feel this, the solution is to have the front brake disc's machined true again on a brake lathe or if they are to thin replace them.