Air in the hydraulic system piping
In order to discharge the air in four rounds, you should pay attention to the action essentials of the discharge (after you step on, you need to block the oil outlet before releasing the pedal).
If the master pump is designed with air screws, air must be vented.
If there is a balancer in the rear axle, air should also be discharged.
Vacuum pump ejector pin and master cylinder clearance is too
Disc type pump fixed guide rod worn or stuck
Remove the wheel and step on the brake to check if the disc sub-pump will shake. If it shakes, the guide rod will wear or unilaterally jam.
The main pump does not produce oil
If the oil pump hose is not smooth or bent, the oil will not affect the oil.