what causes the rear tires to lock up?

When do they lock up?Just when you step on the brake?Driving down the road?Did just the rubber line break?Did the brakes lock up before replacing the brake line?Also you have drum brakes?The more details the better.

as i was driving home from my childrens wic appointment i stopped at mcds and that is when it started locking up the next day i couldn't leave for work because it was locking up so bad. but when i push on the brake pedal i still have brakes. they didnt start til after the brake line was replaced.. but also there is a "rod" if you will that connects from the tire to the frame that is broke would that possibly have anything to do with it? not sure how long its been broke though.... yes i do have drum brakes. when they lock up is while i am driving it.

So you replaced a rod not the brake hose itself?Who replaced it?That rod is to keep the wheel from moving forward or backwards.It has nothing to do with brakes themselves.Your going to have to have the drums for being warped the ebrake cable for being frozen and the rear brakes in general.

the rod was not replaced it is broke and not sure how long it has been broke the brake line was replaced and my dad is the one that replaced it so what exactly is making my tires lock up?

Can you upload a picture of the rod that is broken. I trust saturntech9. If he believes it is a tracking bar or something similar, it can be dangerous.

If its the rod that runs from the body to knuckle you need to stop driving it and replace it before you snap the strut or knuckle off the car.Also you will have to inspect the rear brakes and all the componets to see why there locking up.Are both wheels locking up or just one wheel?There brakes could be adjusted too tight warp drums brake fluid on the shoes etc.