Even when I turn off the manual valve to back yard sprinkler system few sprikler heads are showing small valter flow?
If the manual shutoff is a gate valve, sediment builds up inside and keeps the wedge shaped gate from bottoming in its slot. A slight leakage is normal in these type valves. Remove the top, use a piece of aluminum bar (soft metal) to disturb the sediment and flush it out. Reassemble. It should leak less now that the gate wedges against the seats instead of bottoming out.

Globe type shutoff valves have a rubber/plastic washer that contacts a seat inside the valve. This type shuts off completely when closed unless the rubber washer or the seat have been damaged. Remove the top, feel around for damage on the seat and inspect the washer for cracking. Replace as necessary.

Quarter turn ball valves usually don't have issues with leakage unless there's been some frost damage that's disrupted the valve internally. Replace the valve.

If you're using the manual function on rubber diaphragm control valves for the shutoff, any number of things have gone wrong, dirt, diaphragm damage. Dismantle and rebuild after obtaining a repair kit.