Ok so I'm trying to work on stage 0! I have a 1996 850 R w/ 160K on it. Thought boost was low, confirmed only 6psi boost! Tested waste gate setting ok at 4psi gate starts to open. Figured I would test the bypass valve tried to pull vacuum on vac hose did not hold any vacuum. Pinched off the hose at valve and tested the hose it is ok. So I'm thinking the diaphragm for the bypass valve is maybe bad/ torn. Is that a proper test and am I headed in the right direction? Maybe turbo control valve malfunction? Stuck on the low boost issue at this time, no check engine light and the car runs fine! Thanks in advance!

 

It's possible. Easiest way to tell is to pop the hat off the bypass valve diaphragm and have a look. New diaphragms are cheapish and readily available in Epay these days.
Wastegate starting to open at 4psi seems a little low? One reason I like the MBC's is they hold the wastegate closed till they hit the exact opening pressure. Even if you don't set them to increase boost, they add more midrange torque and help with spool speed.

 

The byass might be bad. My Yellow T5-R only boosted 7 PSI when I got it. Everything was perfect with it as well. These are supposed to boost 10-12 stock but not all cars do. I think Volvo played with the ECU'S at times tricking people.But that is just me.

 

Nope- the wastegate is supposed to start opening at 4PSI on T5 and R models. Note that the boost control valve/ turbo control valve will still keep pressure from going to the wastegate until the computer says it's time. You want the wastegate to open fully when the TCV commands it to, so starting to open at 4 PSI is correct. Remember, unless the TCV is failing, (common issue) no pressure goes to the wastegate until it's needed, so the 4 PSI opening point is appropriate. LPT cars have wastegates that start to open at 2.6PSI!

All a MBC does is replace the computer- commanded opening point with a manually set one that overrides what the computer calls for. The combination of computer + TCV= an electronic boost controller- one that the average person can't adjust.