It's my pleasure, guys, we all share what we know.

Interesting, the hiss is definitely brake related (as it happens when the brake pedal is depressed) but the vacuum hose checks out ok. Plus it seems to be loudest from the inside of the vehicle. It's right that the hiss would be different when the vacuum hose is disconnected (as there's no more vacuum) so that part's ok.

Well, let the mechanic have a listen - I'll put forward my guess that it's the brake booster itself leaking and it needs replacing. That would be why you can hear it loudest from inside. The brake booster has a hole at the back, towards the driver and going through the firewall, where the foot pedal rod goes in. So I'm guessing the rubber diafraghm that's supposed to be air-tight needs replacing. Brake booster is #6 here http://www.ebay.com/itm/NISSAN-472109N0 ... 1116171148 - I'll further guess that you'll need to replace the entire part

BTW, to keep the brake pedal depressed while you pop around to the engine bay, just cut a piece of wood to the right length and jam it in. In the olden days a cut broomstick would serve as cruise control, too! LOOL

Hey PR!!

Thanks again for your reply!
I've been really busy this week so far so haven't had a chance to take it in but I shall do asap!

Just as a note... I saw another post from someone who explains exactly the same problem as I have (replies also suggested the whole brake booster needed replacing), but he found that he "noticed an o-ring on the pushing piston, just when it´s entering the booster, looked a little torn"I wonder if this may be a similar problem for me- though his car was a chevrolet!
Anyway there's no way of telling until I get it properly looked at, I'll let you know what the result is when I find out!

Thanks again!

Yup, something like an o-ring, but not that simple. It is a rubber part that's used to seal the air in.

Basically, the foot pedal connects to a metal rod that slides in and out of the back of the brake booster. The booster is air-tight, so there needs to be some sort of rubber, or flexible, part that seals around the metal rod.

But I think you'll be compelled to change the entire booster - you can't just replace the rubber part.

See the diagram: where it says "valve rod", that's coming from your brake pedal. Where it enters the valve area, whatever rubber is there to prevent leakage, is prolly cracked, and that's where your hissing might be coming from.



Just trying to help from your description on the internet, so forgive me if it turns out to be something else entirely LOL

See also where it says "front vacuum seal"? If the hissing were coming from the front of the booster, that's where it would be leaking.

Hey! Thanks again PR

Of course, I totally understand- I really appreciate your help though!!
I'm waiting to get paid again before I can do anything, and have been too busy to take it in for inspection but as soon as I do I'll update with what it was!

Are you planning to attend any of the trips coming up?

Enjoy your week!