Sounds like a vacuum leak on the booster.

Have you checked the hose/connections?

It's normal to hear the hiss when it operates but it shouldn't be constant.

 

It is normal to hear a small hiss if you press the brake pedal with the engine off. Mine will hiss for about half a dozen presses before the vacuum in the servo has gone.

There is a one-way valve to keep a vacuum in the servo when the engine is not running, but if it fails you can get the hiss when the engine is running. If you get the hiss only when you press the brake pedal it is probably as Alb says a leak in the servo or vacuum line or the one way valve.

 

The hissing stops when i press the brake, its constant say when im sat at lights or not pressing the accelerator when coming to a stop or just lift off the pedal. I'll try and record it later

 

I'd check the one way valve and all of the pipework for splits then. I don't think the one way valve is serviceable. You might be able to check it by blowing and sucking from both sides to see it it is working. Constant blow and suck to see if it holds.

 

Where is the one way valve?

 

Hope this works

 

Is the running of the engine affected?

That would be expected for an induction leak.

 

I dont notice any affect on the engine, what sort of effect might it have?

 

It would affect fuel trim but whether the effect is noticeable would depend on the degree of the leak.

In your case however, if the line feeding the booster is the source of the noise, you may be able to track it down using the Mk1 Lughole.

Failing that, if you disconnect the line at source and plug the hole, you will be able to rule it out/in conclusively.

The vacuum line is quit expensive to replace BTW.