For cars still running the stock emissions and sequential turbo control system, you have a ton of vacuum hoses, solenoids, and checkvalves under the intake. IF these have never been replaced, you can bet that most of them are hardened, maybe even cracked and leaking. This could lead to poor boost pattern/response or even running issues due to vacuum leaks at the engine.

For this reason, those keeping their systems stock/intact should do a hose replacement job anytime the engine is apart. I go through and systematically replace each vacuum hose piece while inspecting for broken nipples on solenoids, checkvalves, etc. I normally use regular rubber vacuum hose for the job, but many choose to use upgraded silicone hose which will never harden, but do tend to tear easily if you work on the engine in the future.

At the same time I recommend replacing, or at least checking, the stock check valves for proper operation. There are aftermarket viton check valves available that are engineered to withstand heat and age without failure, and I offer those as well.

Finally, I recommend that all the solenoids be checked thoroughly. To do this, I heat them up in an oven to around 275*F to simulate engine bay heat. While still hot, I use test leads and vacuum hose to do blow-through tests checking for proper operation.

If all these steps are followed, you're almost guaranteed proper operation of the sequential boost control system. The only other parts that could cause a problem would be the actuators, which rarely fail.

Note that those with single turbo conversions or nonsequential twin conversions need not be concerned with any of this.