Just switched from a TH350 without a transbrake to a powerglide with a pro tree brake. Building boost is not a problem, with the 2step at 3000rpm I can build 4psi on the T-brake and it holds fine. If I set the 2step any higher after it starts popping on the 2step the blow off valve will flutter and open and boost drops to 0 and then builds again. At 3400 it will build 5 psi, at 4000 it will build 11 psi, and at 4200 it will hit 15 psi but at anything over the 3000 rpm the BOV opens right after it hits the 2step. The BOV works flawlessly at all other times, I would think the boost pressure would hold it closed without issues. The BOV is plumbed to the intake manifold just under the carb. Has anyone had this issue or is something screwed up. It is a tial knockoff 50mm BOV.
434BBC, twin MPT70s, Tial 38mm wastgates. If more info is needed just ask. Thanks, Shon.

Didnt get any response in the turbo tech section, maybe someone here can help.
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can you put a stiffer spring in the BOV, or shim the current one? It may solve your problem.

 

Is it a single port BOV or dual? I know some are dual port and after some experimentation i realized they work the best as a single port hooked up after the throttle plate. On a dual port hookup where one line is pre throttle and the other after the throttle plate, your pretty much relying on spring pressure to hold it closed under heavy boost since the diaphragm is seeing (more or less) equal pressure on both sides at WOT. With one port, its guaranteed to stay closed as the diaphragm is much larger than the poppet valve and more boost holds it closed even harder.
Hope this helps!

 

It is a single port. The spring is already stiff, I would think the boost fed to the BOV port would hold it closed no matter what but it does not. The boost/vacuum line comes off the manifold just under the carb.