This may be a repost if so, oops, sorry,
here the issue had stiff brake pedal after replacing my master, heard hissing while applying brakes, so replaced the booster bleed system checked pedal for boost, so car off pedal is rock hard all ok there start car pedal drops a hair, shut car off with foot on pedal booster pushes pedal back up, so go on test drive and there is still very little bite, this is a non abs car 85s3 can't lock up the brakes in a panic stop, also replaced vacuum line from booster check valve due to it being very soft. So what's left the booster check valve.