Shimano mountain bike rear brake set up problem

I m currently trying to set up a Shimano BR-M453 rear can ride brake .... which is situated under the chain stays for some unknown reason - just to make it a bastard to work on .. ? - and I cannot get the arms to stay sprung out .... there’s a 6mm Allen key with which to turn them but only a plastic cap which doesn’t seem to hold them ( away from the wheel under tension ) .... I am utterly flummoxed having not really paid any attention when dismantling them to give the bike a serious clean and service .....
Has anyone had experience of these brakes and there set up .... ?
Any ideas or knowledge most welcome .... two days in and numerous takings apart and I’m fed up ..... is it something dead simple I’m missing ... ?

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Not sure about these ones but on many of the bmx u brakes you have to make sure you put the springs in the right way around. Often one side is silver and one black for this purpose.

Just push arms out wide then tighten bolts. The springs don’t engage\spread arms until bolts are tightened so they just flop about until then. Can scan & ping you reference sheet for these later.

Here you go.

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Hi , cheers for replies ... in the end , worked out the method as forwarded by persona non grata .... I knew it would be something as simple as that ....
Less beer during bike maintenance might be my lesson ....

@may That looks like an MBK Hi-Tech Ranger to me! It was my first proper mountain bike back in 1989 :)

Ha , yes - correct ... I had it a million years ago - gave it to a friend who rides it all the time ...I service it occasionally and keep it going ... but it’s always an awkward bastard .... although I really enjoyed riding it around to block a few times on test drive .... rolls really well - bit to garish for me though
Did see an old boy riding the racer version - same colour scheme - on Sunday ...