What should I do if the liquid level in the U-tube of pressure gauge is uneven?

If there is no pressure, it may be that the rubber membrane or catheter is leaking.

Pull out and reconnect the rubber hose connecting the U-tube. Because the liquid level in the same air is uneven, it means that the pressure inside and outside is inconsistent.

Or take your hand off the film of the metal box. It may be caused by finger pressing.

The liquid pressure gauge consists of a metal box, a rubber hose and a glass U-tube (filled with liquid). The opening of the right tube of the U-shaped tube is upward, and the left tube is connected with a metal box with a rubber film through a rubber hose. When the rubber film of the metal box is not compressed, the water surfaces on both sides of the U-shaped tube are horizontal. (a) The liquid level on both sides of the U-shaped pipe is initially flush, which means that the pressure above the liquid level on both sides is equal. (b) When the rubber film on the metal box is pressed, the liquid level on both sides of the U-shaped tube will have a height difference. The greater the pressure, the greater the height difference. Therefore, the measured pressure can be known from the height difference of the liquid level in the two pipes.