I have a 1962 Swb series 2a with Lwb brakes and I need a new master cylinder. My problem is that my friend has the same setup but has a different master cylinder, which master cylinder should I used, the Lwb one of Swb one? Cheers

you should use the one to match your brakes(lwb), there are (to the best of my knowledge) at least two different master cylinders (cb and cv) and then there's at least one more different master cylinder for the later S3 vacuum assisted pedal box with the dual line system

folks could do with a couple of good photos to help you identify what one you have

Different versions can be seen here but these possibly Britpart shop with care , you want good quality.
Series land rover master brake cylinder | eBay

The CB type has a massive nut on the end like this:
520849 MASTER CYLINDER CB TYPE | shop | www.lrseries.com | L. R. Series

The CV type doesn't and has one of the ports at an angle.

New CB cylinders are eye wateringly expensive and britpart ones are to be avoided. If you do have a CB type you can change it for a CV. You'll need to do a bit of re-plumbing as the ports are the other way round and a different fitting, also the push rod is longer so you'll need a packing piece and longer bolts to fix it on the pedal box. If you're doing this, give me shout and I can dig out the fitting sizes and part numbers from when I did it.

This is assuming you're still using single line non-servo brakes, but as it been changed, you could be using anything. As NiteMare says, post some photos if you're not sure.

i think new seals for my original lwb CB cylinder cost me 4 or 5quid, which being as the bore and innards were fine was a very cheap repair and so far have given me two years of trouble free braking (that's done it )

Anyone for a diagram for the CB? I'm having problems with mine not filling. It seems the piston doesn't return the whole way when pedal is released. There is a metal cup that sits in the top of the piston between it and push rod. When this is removed it seems to work fine.