In the process of making a hydro pump, turns out that what I thought was a clutch master cylinder is a 0.7" brake master, although it was being used as a clutch master.

What rebuild kit do I use?

Looking at the pics on the minisport page ( http://www.minisport.com.au/category9_5.htm ) looks like I have a combination of the kits ie groove & no groove on body !! looking at the existing setup I will need the cup & ring seal (on piston) but the seal at the top of the spring is the one in question.

Is there any reason it won't work for my hydro pump

The ones with the groove have two seals with holes in the middle.
The ones without the groove have one seal with a hole and one cup seal.
Your one looks like the one without the groove.
You don't really need the valve on the end of the spring.

Is there a good brake place nearby? They will have the rubbers and they will be very cheap, probably only a couple of dollars.

If I was using it for a hydro pump I would just clean up the bore and reuse the rubbers. (actually that is exactly what I did with my pump!)

My first hydro pump was a second hand Clutch cylinder, an Ozzy Lockheed type with the Plastic Tank, made it easy to keep a check on the level. Worked for over 10 years. Every now and then, I'd flush it through with metho just to keep it working. I always wanted to put a bigger tank on it, but never got around to it.

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Thanks for the feedback

what does this valve do?