Mike, The black plug, with the red ring, at the rear of the manifold is just simply that, a plug. I really had not thought about what it was there for.
I've never noticed if the LQ4, in my driver pickup has it or not. My 2500HD does have hydroboost brakes.
The LQ9 engine I have in the '5o, supposedly came out of a Cadillac, Escalade. I don't know if they are equipped with hydroboost, or not. They must, because the PS pump on the LQ9 has the dual return hookups.
I found out one thing about 2001, LQ4's. The throttle body on them is different than one on a 5.3, and is cable pull. It is also unavailable from GM, or, reman from any of the local parts stores. The 2003, up, T-bodys, (electronic), are available from just about anybody. Seems odd to me. I wanted one for my '50s, engine, so I could get the thing running with a FAST EZ EFI. Searched the salvage yards and finally found one cheap, so I bought it. I plan to only use it to get the thing running and see what I've got. Then, probably go for a FAST manifold and a big mouth T-body. Since the things are so unavailable, I'll have it as a back-up for my driver truck.
Trying to decide if I'm going to go with the filter/regulator, like you did or full length supply/return, like was stock on this engine.
Haven't been able to work on mine lately, a welding job for my brother, trailer tire change outs, and a troublesome air dryer that required way too much time, took up extra time this weekend.
Steve.