There are two basic issues involved: Assumption of risk (on your part), and the effect of any liability waiver agreed to by you.
Assumption of risk generally means that you can't undertake an activity, get injured in an obvious way, and then sue about it. By undertaking the activity you assumed the risk of that activity. In this case, for instance, you may - may - have assumed the risk of injury from flipping your quad (as those darn things flip all the time). It would be much harder, on the other hand, to argue that you assumed the risk of injury from bad brakes - unless, that is, you were told ahead of time that the brakes were bad and you decided to go ahead anyway.
The liability waiver is trickier. The law of waivers varies quite a bit from jurisdiction to jurisdiction, and you should of course consult a lawyer before thinking about any type of legal action - but many states have fairly specific rules about waivers. For instance, for a waiver to be effective, the law might require that it be in writing and signed, or be dated on the day of the injury, or the law might declare the waiver ineffective if you didn't read it (even if you signed it), or perhaps the waiver has to specifically list the specific injury you suffered, or some other language requirement.
The courts in some states don't like liability waivers and look for ways to declare then invalid; in other states they might hold up better.
In short: I would not conclude that you have no rights simply because you signed a waiver, but I would definitily recommend consulting a local lawyer before thinking about taking any type of legal action.