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An o-ring usually (but not always) has a round section and is made of a compressible material such as rubber, neoprene, silicon etc.
A washer tends to have a square or rectangular section and is usually made of a harder material brass, copper, aluminium, steel etc.
It is not about advantages or disadvantages, but which is used will depend on the characteristics of the purpose, so if it is a delicate device at low pressure then a softer material can be used especially if the units are designed with matching channels for the sealing component to "sit" in.
Many times a copper washer can be used - some car sump plugs have copper washers for sealing, others just use machined steel faces...
Just to muddy the waters as it were, there are o-rings called Wills rings which are a hollow metal torus filled with an inert gas, these are capable of sealing at high temperature and pressure. One use was as part of an engine head gasket sealing the head to the block around the combustion chamber...
R8philSA said: Click to expand...
Hi Phil,AFAIK it is a "fully change rubber parts" job. I have done it twice now, on the 4CV and on the R8 and both times I coordinated it with a serious brake overhaul.So all wheel cylinders/callipers were cleaned, polished and resealed, and I fitted a new master cylinder kit and new hoses. With the R8 I finally threw the pressure limiting valve away and replaced it with a simple tee-piece (there's a thread on this somewhere). I think, but don't know for sure, that if you washed out hoses with meths and really got rid of the older fluid it might be fine but hell, new hoses aren't a bad idea!The 4CV brakes weren't touched for 18 years after putting in silicone fluid in 1990, until I replaced front wheel seals and a master cylinder kit. In that time I didn't touch the brake fluid once and didn't need to.I keep fluid from bleeding and filter it for re-use. Again no problems encountered re-using this fluid.I wrote this up in an article in Fourword a couple of years ago I think.Now, traps!1. The silicone fluid is prone to entrapment of fine bubbles and therefore, if aerated by careless handling and rough pouring, it may be hard to get a good "pedal" (my R8 is a little squishy and I intend to rebleed sometime).2. Things must be really DRY as water will sit in the bottom of wherever it is and may corrode things.3. Silicone fluid seems harder to seal (heaven knows why) so use new copper washers and really get things right at the wheel seals.4. Remember to flush meths through all the rigid pipework and the reservoir. It dries out Over a day or so or you can blow air through.Given the issues with corrosion behind calliper seals, I'd never use anything else but silicone fluid again.Cheers