As mentioned above, you need to ‘match’ the existing pressure. The ease of doing so depends on the pump you’re using (a bog standard would take more than a Hills, for example), the bore/length of the hose (dead volume), condition of pump seals, any slight leaks with the probe etc and the starting pressure (so the higher the initial pressure the more it takes to get the gauge moving).

I must admit I primarily use a cylinder these days, but take the Hills for topping-up if need be. A shoulder injury a few years back means I don’t really like using the pump if I can get away without.