04 srx, replaced booster 3 times the. Current one is the first remained unit, replaced master cylinder. Still have no boost. Replaced vacuum hose and check valve, no change. Measured -18 mmg vacuum..BUT. When the vacuum gauge was connected the car started stumbling and trying to stall. Reconnected the brake booster and it settled down. I am at a total loss. I have a hard time believing thati have a third defective booster, but can't think of anything else to check. The only code is po174- lean bank two, which I've had for a while. Any suggestions?