inputHidden and disabled form fields are lost

I am updating an object with a JSF form. I got some inputHidden fields and
one selectOneMenu which is disabled when I update (instead of adding) the
object. I got some required fields. If I clear the content of a required
field and submit the form, the values of inputHidden fields and of the
disabled selectOneMenu are lost in the response. If I just update some data
without omitting a required field everything works fine: so the setter
methods seem to work but when rendering the response after validation
errors, the getter methods for the inputHidden fields return null.

Any suggestions what I could do?

I am using:
myfaces-impl-1.2.0.jar
myfaces-api-1.2.0.jar
tomahawk-1.1.6.jar

Regards
René