gets air lock,loses power 1999 ford transit desial?

I have replaced the fuel pump,injectors nobody can pinpoint the fault.It gets an air lock causing loss of power,reduction in speed , when I bleed the air off its ok for a while(various timescale).The fuel lines are ok,I have drained the tank to check for water deposits.I have now been told to replace the brackets that hold the fuel filter in place,can anyone help or suggest a solution