I agree with most here - once you buy one you'll find you use it nearly every day! Since buying mine I dont know how i managed without one now....

In my opinion it also looks a bit more professional using a dedicated piece of equipment rather than a 'Heath Robinson'-cobbled together system when the customer is looking over your shoulder - especially when you are charging them for the test!

I prefer to show the customer the leak with the kit all connected up and then they are more confident with the diagnosis and happy to pay any test charge.