A while since I done a h/lamp on these and can`t remember taking bumper off,I take it is the footbrake which is inefficient on your rear barkes,if they are bled correctly and adjusted ok with no seized wheel cyl`s and you still have problems,I do remember problems with the brake pressure reduction valves on vauxhalls to the rear brakes,they are quite expensive from vauxhall and if that is your problem then I would try a scrappy to save some cash.Good luck
cdc
March 2013