Troubleshooting sprinkler system pressure
I have found broken pipes by using a dowel pressing to the ground then pressing my ear to the dowel, if there is water ruling you can hear it 5/7 heads is not that many and if there are no isolation valves it probably worked fine in the past, being built in the 60’s it is probably galvanized pipe, there are many possibility’s for low flow.
The first a broken pipe you have thought of, galvanized pipe also can corrode and build up on the inside limiting the flow of this is the case the pipes will need to be replaced and the pipes are old enough for this to have happened, the third thing I think of is the sprinklers may be plugged with sediment (or possibly a screen filter in line is plugged) so there are a few things to check, you stated you don’t know where the well is you can run water inside the home and use the dowel method to find the pipe (normally the well casing extends above the ground but I have seen a few that were buried (not good). Hope this gives you a way to figure out what the problem is.