Low vacuum at times at idle

[Hmh800]

Hi guys-
I went stage 3 on my 02 avant about 3k miles ago. Rebuilt engine with all new seals, gaskets, etc. and BW k04. Car has run great, pulling 19 in at idle. Just in the last couple of weeks at times it only pulls 13 in and no brake boost. Been a bit hairy a couple times. No CEL. Vacuum returns after a couple revs. One of the few items I didn’t replace on the motor at rebuild were the green check valves. Any ideas where to look?
Thanks!

[Silverex]

I’d check the brake booster hose for any cracks and check/replace that check valve.

[Hmh800]

Makes sense- I’ll take a look. Thanks

[Saddrs7]

The drop to 13 inHg and loss of brake boost definitely points to a vacuum issue, especially since it returns after revving. That usually means there’s a leak or a failing component that’s not holding vacuum consistently.

[Saddrs7]

The green check valves you mentioned are definitely a good place to start. They get brittle over time, and even if they seem fine visually, they can fail under load or heat. I’d also check all the vacuum lines around the brake booster and the N249 system. A small split or loose connection there could cause exactly what you’re seeing.

[Saddrs7]

If everything checks out there, it might be worth smoke-testing the system to spot any tiny leaks. Since there’s no CEL, it’s probably not tripping sensors, but definitely still affecting driveability. Let us know what you find curious to see how this one turns out!

[Hmh800]

I was pulling into my driveway over the weekend after a 60 mile highway trip and it happened again- no brake boosting and I could hear the air leak. Opened the hood and the leak was in the brake boost hose where the hard plastic line goes into the rubber boot at the firewall. It was just slightly dislodged and as soon as I touched it it seated itself and leak was gone. Didn’t have more time to investigate further.
This has me wondering what these cars do for vacuum storage? I don’t remember seeing a vacuum reservoir, but being turbo cars don’t they need one?

[TwoSnelz]

There is a vacc canister in driverside fender area. It connects to the 4 way near the FPR.

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