When do triggers fire vs data checks?

I am trying to upgared a third party application that logs data inot an Oracle 7.3.4.5 instance.

The new application is getting ORA-1401 errors, and thanks to some very kind help from dbock_cdo_at_hotmail.com I was able to determine that the new application is pading a text filed with extra spaces on the end.

So I put the following trigger in place to try to solve the problem:

/*

        Create Triger For MWH_LOG */
create or replace trigger MWH_LOG_trig
before insert on MWH_LOG for each row
declare

   l_date date;
begin

   l_date := :new.dstamp+0;
:new.dstamp := l_date;
:new.brkr_id := rtrim(:new.brkr_id);
:new.brkr_id := ltrim(:new.brkr_id);

end;
/

Heres the table structure:

SQL> describe mwh_log ;

 Name                            Null?    Type
 ------------------------------- -------- ----
 DSTAMP                                   DATE
 BRKR_ID                                  CHAR(20)
 MWH_READ                                 NUMBER
 BRKR_TYPE                                NUMBER(3)

SQL> quit

The filed BRKR_ID is the one thta's causing the 1401.

However, I am still getting the 1401 error, even with this trugger in place!

This doesn't seem to make sense to me. Are the sizes check _before_ the trigger fires?

How can I work around this?

 bind 1: dty=3D1 mxl=3D128(79) mal=3D00 scl=3D00 pre=3D00 oacflg=3D01

    <=3D=3D BRKR_ID
   bfp=3D400a9a38 bln=3D79 avl=3D79 flg=3D05

   value=3D"11X                                                          
                  "