Subaru mushy brakes?

Update: I bought an old Subaru with mushy brakes and replaced the fluid and bled them. The pedal pressure is now quite good with the motor off. However, when I start the motor I can feel the pedal pressure reduce immediately and the pedal sinks. What is going on? Obviously it's something with the brake booster, but... show more I bought an old Subaru with mushy brakes and replaced the fluid and bled them. The pedal pressure is now quite good with the motor off. However, when I start the motor I can feel the pedal pressure reduce immediately and the pedal sinks. What is going on? Obviously it's something with the brake booster, but it's not the notorious weak and bendy firewall. I think other Subarus I've had did this too, but my old Toyota is not nearly as noticeable.
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