Brake cylinders

British cars of the 60s up to the 80s at least were pretty much exclusively supplied by Girling (British) and increasingly Lockheed (USA). Due to the massive market the parts from both firms were supplied in vast quantities, and of course aftermarket suppliers also made huge amounts of pattern parts to fit, so today the supply situation is plentiful. Typically, a seals kit is not a lot cheaper than a fully assembled cylinder, so many say buy the whole cylinder and save yourself the hassle of refurbishing an already old one.

But there is another, better reason not to refurbish an old cylinder. In normal use the piston and hence also the seals only travel over a limited part of their full possible range. As water gets absorbed into the brake fluid and the pipes etc, the exposed inner surfaces of the cylinder start to corrode and get rough. No matter how much you think you have polished the inner face of a used cylinder, these "little used" regions at the extreme ends of the piston travel will be rough, or pitted, or less than perfect in some other way.

When you subsequently use your refurbished cylider, if you do an emergency stop you will apply far higher than normal pressure to the piston, pushing it into these less than perfect outer travel regions of the cylinder. So at the very point when you most need the seal to be perfect, you are pushing it into a region of the cylinder where it is least likely to be so, and where failure is then most probable.The seals sets were originally offered for economic reasons, back in the day when many folks could afford neither complete new parts nor the garage costs to have someone fit them. Today, when in real terms all manufactured goods are perhaps only 10% of their cost 50 years ago, it really does not make sense to cut this particular corner, even in the pursuit of originality.