a/c system is pretty straightforward.
compressor has two o-rings on the manifold. unless your compressor has been changed... then it could have spacers/manifold/god knows what else...
the big lines go to a fitting on the condenser that has an o-ring, out of the condenser there is a fitting with an o-ring, from there it goes to the dryer with an o-ring and three small o-rings for the sensors and the line out, from there it goes to the expansion valve with an o-ring, then out of the evaporator back into the expansion valve with an o-ring, then the large line has a connection in the firewall with an o-ring, and if you have a 123, another line with another o-ring, if not, then the line goes right to the compressor on a 126.
other systems will be similar.
also the R4 compressor has a valve on the back of it, with a snap ring, that could leak also, with an o-ring under it... if it's not leaking don't muck with it...