A Guest Post by E. Fink
Typically, this sort of question comes my way before Pesach. "Rabbi, what's with all these extra chumros that everyone does nowadays, this kitniyos and gebrukts are making me crazy?".
It is important to understand that kitniyos and gebrukts are unfairly lumped together. They are two separate minhagim with very different halachic underpinnings.
Briefly, kitniyos was a gzeira that ashkenazik Jewry accepted upon themselves for a reason. My understanding of the custom was out of legitimate fear that the flour of legumes and non-grains would have traces of "grain" flour mixed in them. It was acceptable in ashkenazic countries to use the same sacks for all types of flour and buying a non-grain flour contained the real risk that grain flour was found in the sack. This is a serious problem and it became accepted not to use any flour similar to grain flour. The gzeria extended to products that were similar to those non-grain flours and thus, the Pesach prohibition on kitniyos was born. In Sephardic locales there was no similar fear and thus no gzeira.
The point is that there was a real, legitimate, rational halachic fear of eating kitniyos at one time. When large tracts of Jews accept a custom as law, there is a not much we can do about it after the fact as far as the halachic process goes. So today, ashkenazic Jews are stuck.
Gebrukts on the other hand is quite different. First, there is no halachic "source" for the custom, it just happened. (See Sefer Todaah) .Further, there is no scientific way for a cooked piece of matza to "become chometz" by adding liquid to it. It is literally impossible. In other words, the not eating of gebrokts is not a custom rooting in a reasonable halachic fear. Therefore, it is not binding in the same way kitniyos are binding. I believe it is even possible to be shoel neder on gebrokts. It is not possible to be shoel neder on kitniyos.
In summary, kitniyos and gebrukts are two very different restrictions. Kitniyos may be irrational in 2010, but it made a lot of sense at one time and served as a protection against a very serious sin of eating chometz on Pesach. Gebrukts on the other hand can be seen as an "erroneous" stringency serving to protect from no real prohibition.
(Having said that, in some ways, kitniyos has taken on a new life and has spawned some irrational chumros as well. But that is beyond the scope of this blog post.)
Search for more information about kitniyos and gebrukts at 4torah.com.