What you have is a Lockheed 5 1/2" servo, not to be confused with the mk2 PBR booster when looking for ease of reconditioning or cost. They're chalk and cheese.
J and L Spares Ltd sell everything you need to rebuild it, or will do it for you.
http://www.jlspares.com/sealkits1.htm
If you don't know what you are doing, don't go pulling it apart with gay abandon until you know what is expensive and what is cheap.
For one, take apart the bell carefully as the diaphragm may be perfectly OK to reuse, don't tear it as they are expensive.
Two, take apart the air check valve carefully as well for the above reasons, these are very expensive as they are a little more than just a diaphragm, they have a swaged in machined part which is a little out of the ordinary.
Three, the chrome rod which is attached to the large diaphragm is hard chromed, and is prone to having the chrome corrode away where it goes through into the body of the booster. If it does and is returned to use like this, it will chip away at the seal it passes through and cause the booster to leak.
Three points to check, and for the rest it is actually reasonably easy (believe it or not) as long as it isn't rusted into a blob.
The three critical parts I mentioned above are all available from J&L for £25, 40 and 20 respectively if you need to replace any or all. If you have taken the unit apart carefully and just need the bare seals for the actual cylinder only, then it's a very cheap proposition at £22.50 and maybe the chrome rod as well for an extra 20.
So, if the booster is in good condition, simply requiring a set of inner seals, and perhaps a chrome rod and a hone then:
22.50 + 20 = £42.50 = AU$70
plus a hone (60??) = AU$130
= AU$Not too bad...
All you need to do then is decide if you want to pull it apart yourself, or have someone do it for you. I would give it to a brake place with the provided spares (you have bought yourself) after ascertaining the condition of the air valve and the diaphragm if I were you, so add an hour's labour for that.
