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

 

25 minutes ago, gear2 said:

no evidence of vac line damage

 

I'd be very carefully double-checking this, both by feel and phone camera images. 

Assuming the same questionable materials are used on your model as on mine; the hard plastic sections of the servo vacuum hose are infamous for splitting at the junctions where they are pushed over hose barbs on things like the non-return valve.  

If you haven't checked such junctions from all angles, have a look/feel now.

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.

Rather than trying to think about it, just do the free and simple check I've suggested.

A new servo is a whole heap of cost and effort compared to replacing a vac hose, so check whether the vac hose needs replacement or mending first; no matter how hard it is to figure out why it might give rise to the observations.

An (extreme) example of a failing servo hose:

 

2 hours ago, Wino said:

 

A new servo is a whole heap of cost and effort

 

Just to confirm (tiny bit OT) by servo we are talking about the disc/circular thing the master cylinder bolts onto correct?? Rather than the vacuum pump.

That's what I was refering to, yeah. AKA brake booster.