Good Morning Folks, First, i am not really tech savvy with HVAC.

Question: i have a Tafco Walkin in Refrigerator for my restaurant which was purchased roughly 12 years ago. Compressor is located on the rooftop of building and was put there due to extreme heat and dirt in the air inside the restaurant. It was worked beautifully since about 2 years ago. Leak was found inside the box (inside the fan). It was repaired. Since then, i've had to have it charged about every 3 months, as it appears there is another leak. HVAC tech is telling me that it has to do with the valve not opening for the Gas to get into the system (Low pressure gauge) It appears that i have a problem when the temperature outside has a drastic change (ie: from 70 degrees to 40 degrees overnight) . This happened 2 days ago when i came into work and the box had a temp of 46 degrees. it hovered around 50-55 degrees for about 4 hours, before pulling back to normal. Am i getting my chain pulled here, as i am getting tired calling a tech to come and spending $ everytime, or is the unit need to be replaced?

I appreciate any insite on this.

Thank You
Robert