Fractional horsepower refrigeration systems typically come from the factory sealed, with no access valves installed. While working on these systems, if some type of access valve has not already been installed, you will need to add one to the system to read its pressures, add refrigerant, or recover its refrigerant charge.
One common access valve initially used is some type of saddle valve. These valves are convenient and generally easy to install, but they are not usually considered to be a long-term type of valve and generally should not be left on a system permanently. They are prone to leaking over time, and it is considered a best practice to remove them after or during the repair process.