The reason why the brake calipers are not tight.

First, if the brake master cylinder fails and the pressure of the brake master cylinder is released, it will lead to the phenomenon that the soft brake can't be braked. Re-bleed the brake system air. Replace the brake master cylinder if necessary.

Second, the failure of the brake vacuum pump, if the brake is hard and there is no assistance, will also cause the brake to stop. Check the pipeline of the brake vacuum pump for leaks and replace the brake vacuum pump if necessary.

Third, the brake slave pump or slave pump pipeline is blocked. If a single wheel can't brake, check whether the oil from the brake cylinder and pipeline is good. If there is no problem with the brake oil pipe, replace the brake cylinder.