Refrigerant is leaking from system

TRUCK VIN #1GBKC4F5VJ113092
TRUCK SERVICE BODY MFG) OMNIVAN
REEFER MFG) TECUMSEH
REEFER MODEL/SERIAL #'S)
M#AJA7441AXA / S #9A72458741

Hope someone can help me with this; haven't been able to get help from Omnivan or anyone else. I service a fleet that has five of these type trucks in it, all with reefers on them. Truck chassis has a Omnivan food delivery body mounted on it and a cooler/reefer unit mounted under the passenger side. The reefer starts cycling rapidly so I check the system pressures and charge it some and it goes back to normal operation. A day or two later it stars rapid cycling again; I recharge the system and use a electronic sniffer but can't find any leaks. Yesterday/two days later it's doing the same thing so I inject R12 dye into the system to see if I can find a leak but didn't see anything the two hours I worked on it. Charged the system back up to normal operation but know it's going to start doing the same thing. I'm not very familiar with reefer systems and need to find a good source of information for these types of systems along with suggestions on the current problem I'm having.