Presta valve nuts are manually operated secondary seals that are not connected to the main valve seal itself. These captive nuts perform the same job as a Schrader valve cap.



Some Schrader valve adapters for Presta valves are known for leaking and, as such, should not be left on the tube. Sometimes your best bet is to carry a Schrader valve adapter with you for "emergency" use with Schrader valve gas station air compressors tire chucks (temporary use of Schrader valve adapters are also often used to check tire pressures if you don't have Presta pressure gauge on hand). However, I recommend that you keep a Presta valve compatible air pump and pressure gauge in your bicycle tool set.