I don't really get it either. What was weird is while hooked to the booster and reservoir I got almost no vacuum in the broken state. When I disconnected those and plugged the hose with my finger or measured directly at the pump I got 20 in HG. That's why I thought my pump was good and the booster was bad. But it was the opposite. After swapping the valve it all worked.

No leaking pipes...believe me I checked really hard before I started this odyssey.

I ended up changing the booster and mc because they were cheap and I wanted to do some front suspension work. Car also has 200k on it.

Oddly the engine sounds different now. The pump seems to have a low pitched growl at mid throttle. I guess changing the check valve makes it work less?

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles