It can take overnight for some leaks to show up.
Actually pressure testing requires inverting the carburetor and applying 5-7 psi pressure and seeing if the pressure drops over 30+ minutes. This is with only the natural weight of the float. Holding the float with your hand prevents proper testing of the needle valve seal seating.
Here I use a MityVac 8500 tester for this. Note the test line will needs clamps to insure a tight seal of the pressure line. This is same tester I use on 2 cycle cube carbs.
Most gravity fed carburetor pop off at around 5 psi and fuel pump versions can go as high as 10 psi depend on the design; more commonly around 7 psi.
The above tester is also very useful in testing 2 cycle engine's crankcases for pressure and vacuum leaks.
