A rack is agreat project! Spend time thinking of what you want it to be for you, there is alot of room here for customization!!! A rack that fits without any adjustable brackets has that "trick" factor!
some thoughts:
-If you go with the brakeline/hydraulic tubing, remember to sand off any galvanized/zinc from the areas you'll be heating, at brazing temps, the zinc burns off and the fumes are toxic.. not to mention it will contaminate the joints.
-No matter what tube you use, the area should be CLEAN!! I sand, then wipe w/alcohol.
-Flux liberally.
-Silver rod flows at a few hundred degrees lower temp than brass, if you can find it cheap or can afford it, I'd say go with silver, its a bit easier, flows better.
-Get a file or two of the diameter of your tubing so you can fishmouth the tubes into eachother. Braze will not fill gaps.. 1/32 maybe max. Spend time getting them to fit right. It will make brazing the joints much easier.
have fun, post pics!!!
~Steve