Bob: Know what you mean about the ferrous pipes, I think every car I've had has had them as standard (ranging from euro boxes to japanese sports cars) and after 10 years or more they go and have to be replaced. Oh well, the whole rear axle's been done now, it's in tip-top condition and all bled, thanks for the help. Think I've just got plain Copper, not Cunifer (said copper on the reel!)
sae: When you put your foot on the brake, weight transfers forward and the back end lightens up, if the bias valve is working, it'll reduce the pressure to the back end (stop the wheels locking up), however if you've got load in the back (like tools or cargo) then it'll keep the back end down and so will require more pressure to lock up the wheels, the bias valve will allow more pressure through (all based on ride height). Easiest way to check if it's working is to get someone to put their foot on the brake and pump it while you look at the lever connected to the valve, it should flex up and down a little. Checking if it's in the right position can be a bit of trial and error - the aim is to have the back locking up at the same time or just after the front (however I use rear bias on my rally car to encourage the back end to be a bit loose)