2015 Forester Touring - hard brake, air hissing from brake at startup

[heatherwirth]

Have a 2015 Forester Touring with 70k miles - was rarely driven previously, I've owned for 1.5 years with almost no issues. Had new front brake pads and rotors installed last month, and new CV axle. Over the last few weeks, brake pedal has been increasingly harder at start up. Functions fine once car is on, but when I press down on the brake pedal while idling in the driveway, there is a clear sound of air hissing. This does not happen while I'm driving. If I take it out for several trips in the same day, the pedal is only hard for the first trip, after that it's fine. It's almost as if it sits for 24 hours, the brake will be hard again at start up. Weather has been mild - 70s and 80s with minimal rain.

Vacuum leak maybe? Is this an asap repair? My regular shop is booked up for weeks, and the closest Subaru dealer is 1 hour away - would a typical shop be able to diagnose and repair? I can do some minor things but this may be above my pay grade.

 

[heatherwirth]

adding -
conducted some tests at home using my Haynes manual, trusty old ChatGPT and some YouTube tutorials. Looks to be a bad brake booster. Waiting to call my mechanic tomorrow - any idea of cost and repair time would be much appreciated.

 

[bmart]

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