Practically Classics Forum

Going to work on brakes of my 79 MGB would like to flush through system unsure if mineral or silicone fluid used, just bought car. what is best to flush system, I heard Meth can be used Any suggestions?

I'd flush through with more fluid if I were you - I'd have thought that Meths would be rather damaging to seals, not to mention a fire risk.

If you want to check if it's mineral or silicone fluid that's in there at the moment (I presume that's what you meant by synthetic?) then take a sample and see if water will mix with it. If it doesn't then it's silicone.

Hi Thanks for that, did,nt want to just flush through with fluid until I knew what was in it,I believe it`s not good to mix them. But thanks for suggestion about mixing with water,( yes did mean silicone.)
Ron

Most likely to be common-or-garden DOT4 in there now.

When I flushed out my Alfa system after a few years I was surprised at how dirty the original fluid was ... On a classic be prepared ... But it is a lot easier to see when the new fluid has made it through.

Dot type brake fluids are NOT mineral oils. Modern Dot type fluids are either a PAG type or silicon.

Silicon is purple in colour.
PAG ( Polythene something Glycol) is amber.

Mineral brake fluid is usually LHM as in Citroën and post 1980 Rolls Royce and some Jags. Green colour

MGB should be Dot 4. To flush just use Dot 4.

If the fluid is purple then only silicon can be used. Once silicon is used in a brake system it can't be changed back to Dot 4 unless the system is rebuilt.

Silicon brake fluid is ok on a road car. But it does need bleeding once a year to expel moisture from the low points.

Mineral oil in the brakes on a MGB will cause the master cylinder seals to fail within days if not hours.

Bob.

History wrote:Dot type brake fluids are NOT mineral oils. Modern Dot type fluids are either a PAG type or silicon....

Glycol isn't an animal or vegetable sourced substance and that only leaves mineral , I understood precisely what Luxobarge meant by mineral, which suggests that others do too: The use of the generic term "mineral" as opposed to a synthetic base, such as shampoo, Cuba's main type of brake fluid.

DOT 3, 4 & 5.1 are all based on same ingredients just better boiling points. DOT 5 is not and used in those old Citroens and shouldn't be mixed with the others.

DOT 3, 4 & 5.1 are also paint-strippers .. which is why the area around older master cylinders is often paint free as it gets spilt when topping up. And attract moisture so should really be changed every couple of years.

I use Castrol Response(?) in my Alfa and that is a very light coloured fluid. High boiling point and less prone to moisture uptake. Recommended to me by a Lotus mechanic.

Nope, DOT 5.1 is silicone, and one of its main attractions (for me, anyway) is that it does NOT damage paint.

Luxobarge wrote:Nope, DOT 5.1 is silicone, and one of its main attractions (for me, anyway) is that it does NOT damage paint.

Sorry, no, its not. Dot 5.1 is the non-silicone version of Dot 5.0.

Dot 3, 4, and 5.1 are all "mineral" fluids.

Flatlander wrote:

Luxobarge wrote:Nope, DOT 5.1 is silicone, and one of its main attractions (for me, anyway) is that it does NOT damage paint.

Sorry, no, its not. Dot 5.1 is the non-silicone version of Dot 5.0.

Dot 3, 4, and 5.1 are all "mineral" fluids.

Sorry my bad, you're quite right - I was getting my 5 and my 5.1 mixed up (as, of course, you should never do!). It's down to the effect age has on memory.....

Glad we got that one sorted out.

Cheers!