I have a toyota four runner and when I'm going down the hill and hit the brakes it shakes, but if I'm running in straight road it doesn't why?

The extra pressure, and possibly heat generated, going down hill to stop magnifies a lateral runout (LRO) problem. My guess is that the pistons in the front calipers are not floating properly and hanging up, due to not changing the brake fluid every 2 to 3 years. Consider replacing the calipers, pads, and completely changing the brake fluid. Be sure to remove any rust between the wheel and the hub, which contributes to LRO problems. This link should help:

https://www.askmehelpdesk.com/cars-t...ml#post1618425

Another possibility is that the lower ball joints and/or outer tie-rod ends are shot. Have a qualified mechanic go over the complete front end. Most front end suspension parts (e.g. ball joints, wheel bearing/hubs, outer tie-rod ends, and struts) need to be replaced every 100,000 miles. Big money is saved in learning how to do the work yourself. In the long-run, it's cheaper to get all of the front suspension work done at the same time.

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Originally Posted by yamelis39

I have a toyota four runner and when i'm going down the hill and hit the brakes it shakes, but if i'm running in straight road it doesnt why?

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Along with what Txgreasemonkey posted, there are different loads you put on your car or truck while braking on a level surface as compared to going down hill... : On a flat, straight braking, most all of the braking effort is generated by the front brakes. The rear brakes to a little but not much. Now, during a down-hill braking maneuver, because the vehicle is loaded differently, your braking will be spread out more and the rear brakes are applied more to keep the vehicle straight.
I would recommend a complete and comprehensive brake inspection. Make certain the brake rotors and brake drums are properly measured. As these two critical component wear, they will be less able to dissipate the heat that builds up during a braking event.