in fact speedfit connectors actually use the water pressure to help create a watertight seal. as the pressure pushes against the connector the fittings inside bite harder onto the pipe, keeping them firmly in place. the seal itself is achieved with an O ring.
when the pressure drops they cam relax a little, so it sounds quite normal. of course if the pressure is exceptionally high they could leak but they are designed to withstand mains pressure and can probably take a good deal more than your gravity-fed system can deliver (caveat of course, not knowing the kind of pressure you are getting). if you are concerned you can lessen the pressure by part closing the service valves at the bath if they are fitted.
i should point out that i am playing devil's advocate here - i am not a fan of speedfit myself and don't use it, especially in inaccessible areas. good old solder and compression for me! call me old-fashioned...
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!