1) First, check the water tank and make sure it’s pushed securely into the mop.
2) Next, take a look at the cap and make sure it’s not cracked.
3) Check that the rubber gasket at the tip of the cap is still attached to the cap. Are you using anything in addition to water in the tank? Vinegar will cause deformity of the gasket. Other solutions may cause breakdown of the gasket and/or other parts of the machine. Please be sure that you put only water in the tank. Do not use vinegar or any other solution in the tank.
4) Check that the x-valve (located inside the mouth of the cap) is working properly. Push up on the valve over the sink to ensure that the water releases out of the tank. If there is an issue, the cap assembly will need to be replaced. You can do so by clicking here: http://www.bissell.com/parts/?sc=part&productid=23228&partid=23534
If your mop still leaks please call us at 1-800-263-2535, Monday - Friday 8 a.m. to 10 p.m., Saturday 9 a.m. to 8 p.m., or Sunday 10 a.m. to 7 p.m. ET. A representative will be ready and waiting to help you out!