SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

The valve to that zone more then likely has debris inside it that is preventing it from closing all the way. so water is seeping through and coming out of the last sprinkler head

Find the valve to that zone and take it apart- clean the diaphram, solenoid and flush the body- pull any large debris out with needle nose pliers. Reinstall and see if it holds tight- you can use your water meter's leak detector to help indicate if it's holding tight.

I recommend to replace the diaphram and solenoid ( The whole top of the valve) , and if it's a crap brand valve then replace the whole valve which lets you flush out your main line even more.

Is the problem head located much lower than the other heads? It could be that after operating the zone, the problem head is draining the zone line. When the zones active, all the piping is full of water, when the zone shuts off, it could take minutes or hours for the water in the pipe to drain - at the lowest point. If the zone valve is the problem, shut that valves flow control, (top center handle), off then wait a few days to see if the water stops. You may be able to listen to each valve to determine if water is running through them when their supposed to be off. If you find the valves the problem, you'll need to disassemble it and check the solenoid, diaphram, valve seat and orficies for any blockages, rips or tears. But, if water draining from the pipe is the problem, you can purchase and install a check-valve-in head. These can help stop water back flow up to 8 ft. in elevation. Often, this may require additional check-valve-in heads because other may become the lowest point to drain. This is not uncommon on steep lots or where driveways slope to meet the street, or there are elevated planters that water with ground level sprinklers. The water will syphon to the lowest point.....hope this helped

I'm sorry man but your post pretty bad advice..

He said until recently it has been leaking Constantly

As a technician ( I'm assuming you are) I hope the advice you gave is something you don't follow.

A proper way to tell if the sprinkler system is leaking is to utilize the water meter's leak detector. A 45 second on/off bump test with the back flow assembly will tell you if the irrigation system is leaking.
If there is not movement on the meter then it is safe to say this is drainage from a slope. Although I do agree every last head should be a SAM .. Typically a head should NOT drain for hours on end.. The zone must reach atmospheric pressure until it does and in MOST cases this is much less then 30 minutes- dependent on design and slope head.

Basically what I'm trying to say
Make sure you don't have a leaking valve before you suggest a sam..

I agree, Pressure test the mainline! Sounds like a weeping valve to me.