I've noticed looking at second hand air reservoirs on ebay.

Early ones are aluminium, and later ones are steel.

Were there problems with the aluminium ones or were they just replaced with steel for cost reasons.

Considering the steel ones rot horrendously, replacing the rust one with an ally one seems to be a good idea on paper.

Mines alloy, 2003, and looks like new... I expect it was a cost issue.... Pete
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Mines steel 2004. Even though it's behind a cover had mild surface rust patches on lower surface.
I just rubbed down, repainted in situ and then gave it a coat of grease.
If I had the choice I would go for aluminium purely on the grounds of minimising corrosion problems.

Surely the ally unit would save a few kilos as well.

I expect it would probably be quite thick ally, but then again on a 3 ton car it's probably not going to make a vast difference to economy or performance.

Are but it will make a major difference to JLR, I imagine that they are saving 60% + on the cost by using steel. Plus the steel unit will succumb to the tin worm quicker, leading to enthusiastically over priced replacements 's Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

I've noticed the Ally ones are BMW sourced (made in Austria) whereas the Steel ones are UK sourced, going by the labels on them.

So i would imagine the steel ones are MUCH cheaper.

When i get around to replacing mine i'll go for an Ally one i think.