What causes vibration and skidding tires while braking?

I have a 1997 ford aerostar, and it runs beautifully. The only issue is the brakes. I had the brake lines, calipers and front left wheel bearing changed. The brake pedal goes down pretty far before activating the brakes, and a lot of times if i apply a little too much pressure they don't work and i just start skidding. Also, if i'm going fast and start slowing down the van vibrates pretty violently. What could cause this, and estimate how much to repair?

Perhaps your tires have poor grip. The pedal goes down as far as it has to. Hopefully all the brake lines were flushed and bled. If any air is in a brake line, its effectiveness will be severely reduced. Braking in a car with warped brake rotors will result in vibration or shimmying.

The vibration sounds like brake rotors that should be turned. It sounds like the brakes are grabbing unevenly. Turning the rotors and replacing the pads might clear this up.

The pedal going down sounds like a leak in the lines. Brake fluid is hydrophilic (water loving). Over time, it draws in moisture through all the rubber seals in your brake system, thus causing them to dry rot, thus causing them to leak. Old brake fluid is BY FAR the #1 cause of leaks and brake failure. Change the brake fluid! Most mechanics are unaware of chemistry.

Even without your list of parts, the FIRST thing to look at would be the pads and rotors, which you don't say were even looked at.

Fist, it sounds to me that your brake lines were not properly bled. You have air trapped in the system and this is why the pedal is soft.
Secondly, It sounds like they did not properly tighten the wheel hub back against the newly installed wheel bearing. This is indicated by the vibration and wobble that occurs when you brake. It would also be doing it when you drive at certain speeds. These both are serious problems and can cause you to get into an accident. Please have both matters addressed as soon as possible.

Your brake rotors are warped. The reason the pedal goes down far is that the warped rotors are pushing the pads back into the calipers.
You need new rotors and pads.

The pedal going a long way down is because whoever worked on your brakes didn't do a good job. The brakes need bleeding.
The skidding is down to you. "if i apply a little too much pressure". So don't apply so much pressure.
The vibration is bad rotors.

Can be many things, even a combination of things! If this has occurred since having work carried out on vehicle? Then something is wrong and it needs to go back and get checked. Excess pedal travel sounds like air in brake lines and or incorrect adjustment if we have shoes rather than disc pads. ( rotor pads USA ) touring rotors to a different position is both pointless and in many cases impossible due to fixing screw retain a Rotor. We may have roar warped? Tire wall damage? Excess wear in a joint? Damper failing? Etc combination of these and so on. Brake should Not skid unless we're braking too hard or on loose surface eg. Gravel. Or vert wet with poor tire adhesion /excess vehicle weight /speed. As this IS a safety item you must get it checked out at a competent garage.

Sounds like not all air bled out.
The vibration is either bad rotors, calipers, or bad wheel bearings.
"too much pressure, don't work, skid"? Wrong. They lock up or wheel would Not skid. Could also have broken wire to ABS, if you have it, or air in crossover valve. Or hard ceramic pads you aren't used to.

Didn' t someone Road Test it? Take it Back.

The brake pedal is connected to ALL of the brakes, not just the ONE or TWO that you worked on!

You seem to have MULTIPLE problems.

Have ALL of the brakes inspected by a licensed mechanic. He will tell you why your tire is locking up and skidding and why your pedal goes too low.

This is a dangerous condition and could make your car swerve or spin out of control, especially on wet or slippery pavement.

You have admitted that something is wrong, so stop denying it and get it repaired. As soon as possible!

-General automotive mechanic since 1972

Misaligned wheels?