The 'pig' wouldn't work in your situation
You may want to get a pro in... I've had so many customers who have spent hours doing what you are doing- I swoop in a find the situation in 5 min.
Locating leaks for laterals-
Step 1- Turn off all sprinkler heads / or dig up and cap all sprinkler heads- Utilize the water meter ( if applicable ) and calculate water loss- compensate 1gpm extra for leaking fittings. Let the zone run for 10 minutes or so and walk around looking for a wet spot. This really shouldn't be more then 30 min to 1 hour of work. For most situations this will cause the leak to surface unless it's running straight into a drain. Leaks happen much more often near the sprinkler head then elsewhere.
At this point if you can't find it- more then likely you ARE NOT going to find it and you now need generalize the area of the leak instead.
Step 2- If a wet spot doesn't appear the next step is to isolate- Try to find the middle portion of the the zone , Cut and cap and then utilize the water meter to see if the leak is on the first portion or second portion. A simple slip fix can repair this. Understandably this all depends on the zones design so use your best judgement. Do this one or 2 more times - try to isolate it into a small length of pipe. The best thing to do now is to re-route the pipe and abandon the section of the leak.
Seems like you have gotten to this point? It's best to just re-route that 50' instead of spending more time to find the leak. I use My LD-12 to continue on to find the leak but that's not really available for a homeowner.
Good luck