I notice the hose became hard/brittle and when I bent the hose, you can see lines of cracks. Could this cause boost leakage?
What’s the purpose of the hose from the Turbo to Wastegate?
When the pressure from the compressor housing side of the turbo approaches the wastegates Spring Value(ie. 7 psi Spring) the wastegate will open. This is how boost is regulated. The downside to this is that, typically the compressor will see 7psi before the engine will(usually due to the pressure drop through the intercooler and associated piping), therefore you may not SEE the 7psi on your boost gauge(which usually reads off the intake. HTH.