Problem – overall brake performance not great, ‘nothing happens’ for half a second when you want to brake hard. Not good for emergency stops! Car has been like this since I got it, new brake pads didn’t help much. Brake discs are ok.
Suspected culprit – the brake booster pressure accumulator.
Accessing it is super easy – remove front left wheel, remove wheel well trim pieces (8mm bolts), and you can see it. Before going further pump the brake pedal to get as much fluid out.
You need a 17mm ‘c’ wrench to undo the pipes – they are pretty tight and because the bomb is mounted on rubber bushes it’s hard to get good torque on them. I took them by surprise by giving the wrench a good whack with a sledgehammer(!) – worked perfectly 
Two 1/2″ nuts are used for the mounting, these were not tight and easy to get off. Just pull hard on the air duct to remove it and you have easy access to the rear one.
The obligatory shot of the new piece:
Now just pull the pipes, not much fluid should leak, swap the rubber bushes over, mount the new piece, replace the trim. Top up fluid if required (power steering reservoir).
Results – was looking good soon as I started the car – pedal was smooth and not hard like before. Jabbing the brakes at 20mph like I did before resulting in the ABS kicking in and the car pretty much stopping on the spot (or so it felt 
. Braking performance is now absolutely fantastic – clearly how the E31 is meant to brake!
Difficulty – very easy.
Cost – pricey – $290 is the cheapest I could find the piece versus $360 dealer list price. It seems in the past few years the piece has got very expensive as forum posts from a few years ago talk about it being sub $100. Because it’s pressurised nitrogen these days you can’t ship it by air either and some places won’t even mail it!
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