access control problem

HI guys I am really having a serious access control problem

Suppose I have a certain static path like this(with $may_cache set to true)

path => 'node/add/product/agriculture',
access => return true
............................
..........................(with other options)

Now I am overriding the above path with a dynamic menu(with $may_cache set to false)
path => 'node/add/product/agriculture',
access => false
.........................
.......................(with other options)

I think that the overriding menu path should not have given access to the page but it is.Why is it so??
Actually what I want to do is override the menu and allow the page to display only if a parameter is passed with the url like
node/add/product/agriculture/3 where 3 is the taxonomy term
All other visits to the url node/add/product/agriculture should return false until and unless a taxonomy term is given,

Now the first one given in the above two is defined in another module and I don't want to change it,but override it with my own.But it's not working.How can I do it.

Seeking for help