Thank you for contacting FixYa with your inquiry.
The Rubber Diaphragm by nature is usually somewhat collapsed and twisted looking. Unless it appears to be non functional it's probably OK. It's important to have fluid and make sure the air is out of the system. There is a bleeder screw to allow for evacuating the air out of the system.
http://cyclehuttktm.com/FicheFinderNew/FicheFinder.aspx
Go To:
2004 > 450 EXC-G RACING (USA) > CHASSIS > BRAKE CALIPER REAR
Also look at the repair kit Item #2 (50313081200)
You can purchase a complete repair manual from this site:
http://store.payloadz.com/details/391226-eBooks-Technical-2003-2004-KTM-450-525-EXC-Racing-Owners-Manual.html
Make sure you have serviceable pads, proper KTM brake fluid, and the air is out of the system and the brake cylinder is functional and you will have brakes. There is also an accessory available the cools the fluid for the rear brakes and keeps them from fading if you ride the pedal or are constantly braking. Ask you dealer.
Let us know if we can be of further service.
Kind regards,
TF
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