If you can blow through the bottom pressure line on the wastegate either the diaphram is shot and leaking or the valve guide is worn out. It's difficult or it may be impossible to get parts for the original wastegates so replacing it with a 46mm Tial is the next option.
Around 9 years ago I found an original diaphram for the factory wastegate on the classifieds for around $35. I'm glad I have it in case I ever need it.
I also have a new Chinese Tial copy I never installed and it looks like it's really well made. Especially for only $79. Tial springs fit it too. I put a green Tial spring in it. It even came with a really nicely made chrome plated wastegate tailpipe. I sold the pipe for $50 around 7 years ago.
FWIW, it looks like the website I got it from is still active...
46mm WG Performance Racing Products for International Sales from SSAutochrome Inc.