gear2

The thing that is making me think its not a vac line is the hiss is audible from within the cabin and when i put my hand over the gray diaphragm(?) (not really sure what its called but it covers the rod running into the booster) I am able to distort the hissing noise.

Ive tired searching the forum for a similar issue ( but it doesn't seem to be the same. Long story short - was driving a long, heard a small pop sound, brake pedal pushed out then about 15 seconds later the brakes hardly worked, I could get the car to stop if I slammed the brakes but there was no smooth transition. It was as if the booster wasn't providing any assistance (ie wife couldn't get them to work at all) and there was a constant hissing noise when the brake pedal depressed. https://www.youtube.com/watch?v=BGM8odNDoEo&list=PLjwXNC6y8hB0zn7W6p8gk9akBSeTVdz4o (Basically the inverse of this video - as in my car hissing when the pedal is depressed) Also noted if the pedal was depressed when the car was idling it then engine went up to 1500 rpm. I am assuming the issue is with the brake booster?? Or is it something else (no evidence of vac line damage) - is it the case I will have to replace the whole unit or is there a gasket or seal that has failed an I can replace? (ie something like thishttps://www.ebay.co.uk/itm/282981778656?ul_noapp=true) Cheers for any help