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Hello. Welcome to Just Answer. Please allow me to assist you. Anytime you hit the brakes and the steering wheels vibrates back and forth, the brake rotors are the prime candidate for causing this. There are many reasons that can cause this. Most importantly, if the rear brake calipers are not doing their half of the braking duty, the front brake rotors overheat. When rotors overheat, they tend to flex just a bit. We call this 'warpage'. When a rotor warps (this can be 30 thousandths of an inch or so), instead of being a perfectly straight braking surface, the flex of the metal creates a 'high spot', and a 'low spot'. As the high-low-high passes the brake pads, the friction from high-low-high results in a vibration which is transmitted to the steering wheel, as the steering allows this transmittance of the vibration. An easy way to check this in the shop is via the usage of a dial indicator tool. ALl good shops have this tool. The rotors are measured with this tool, and the 'warpage', if present, will be very easy to measure. If the front rotors are warped a faire amouint (given the fact that they are new", we need to determine two things. 1) are the rear brakes working properly? If not, the front brakes do all the work. This causes the aforementioned overheat condition. A good whop wiull have a laser temperature probe. They should use this tool to measure the temperature of all four rotors after a road test. If the rear rotors are cold, or markedly below the temp of the front rotors, the rear brakes need attention. 2) the front brakes may be staying 'on' whilst driving. Thsi can be caused by damaged front brake caliper hoses or other hydraulic issues. The brake pads may be seized in the caliper brackets, the calipers may be hanging up ion rust, the new rotors could actually be warped right out of the box. Low quality rotors found in auto parts stores are notorious for this. I cannot tell you how many new aftermarket rotors we replaced in the shop...brand new, with less then 5 miles on them, vibrating worse than the old ones that were removed.
The recommendation I have for you on this issue is to get further diagnosis, and isolate the problem. It is rather apparent that there were a lot of parts sold to this vehicle without benefit of a thorough brake diagnosis. I cannot tell you what they are going to find, but they should find something. I would suspect that if the vehicle was fitted with non-factory rotors, that would be the first place to look, via measuring the warpage. I would also recommend the temperature of the rotors after driving it be performed in order to ascertain that the rear brakes are actually working at specification.