i also posted pics
https://forums.saltwaterfish.com/t/297623/pics-diy-pvc-overflow of my setups. with these overflows about a week ago.
a couple of things to help you out. the plans you have are a little old!
a few things have been modified. to make these run better. if you click link you'll see what i mean.
1. the siphon runs better when the bottom elbows in the tank are 2 - 6 inches higher, then the bottom elbows outside the tank.
2. the use of an elbow on your overflow pipe. ( instead of that straight pipe)
will help siphon run better, will quiet it down 99 percent more, help control water level, and help siphon start up faster.
3.on your vent pipe, when using an end cap. a hole must be drilled in center of endcap(as stated above). trust me it will still quiet it down. to help even more put some filter floss in end of pipe befor putting endcap on. use enough, so you dont loose it down pipe, but not to much to clog airflow.
4.ok somtimes with these systems,overkill on pipe size, really makes them run unstable. as they can suck air an slow down, or stop all together.
I.e. a 400 gallon tank would run fine with 2x 1 1/4 inch pipe. in any tanks 125 and under. i would only use 2x 1 inch notthing more.
in this type of system i would always use 2x just incase 1 did stop for some reason, the other will handle the flow!
if overkill is being used then 1 overflow would run more stable than 2. you'll just have to worry about it stopping.
you guys need to trust me! i have been building these for over 5 years,and have done of over 80 tanks with these overflows around the area, over the years. And i have learned for my mistakes.
there has been more mods made to this system, a double and a quad.
i will try to find these plans, and post them and a more up to date plan of the
single pipe, even if i have to fix it myself.
HOPE this helps