REPAIRS PAGE 106
BRAKE FLUID CHOICE – THE DOT 4/5 CONTROVERSY
This subject seems to be receiving a lot of auto press argument at the moment, with
passionate views verging on both extremes.
I’m not going to take any such firm view that could be fatal! But just explain some
background facts, then you make your own mind up!
First the background—
Brake fluid, yes we all know it’s needed, but it must cope with high pressures and
temperatures without attacking the rubber seals.
Traditionally this has been a poly-glycol based fluid originally made from crude oil and
designated DOT4.
The ‘DOT’ rating is from the US ‘Department Of Transport’ and their ratings closely follow on from the US ‘MIL’ specs.
Current DOT4 is synthetic, so don’t fall into the trap that if DOT5 is synthetic then
DOT4 must be inferior crude oil crap, it isn’t.
DOT4 does however have problems (as indeed does DOT5 – more on that later.)
The biggest DOT4 problem is that it absorbs water, avidly.
This causes micro fine rust at the back of pistons and behind seals. This causes
sticking and partial seizure.
Under severe braking the water can boil, end of braking power!
This is why it is a universal recommendation that DOT4 fluid be changed every 2
years, and most of us know this to be the case, but how many of us actually do
it?
Right, so that’s DOT 4, now what’s this DOT 5?
Its development was prompted by the US primarily to avoid DOT4 water absorption
problems encountered in some of their hot and steamy military ‘acquisitions’
such as Panama etc. Its main characteristics are: It’s usually, but not
always, silicone based and does not absorb water to any extent compared to DOT4.
Its boiling point is a bit higher – but not significantly. It’s a lot more
expensive.
This alone is hardly conclusive justification for DOT5, so I suppose the best way
forward is to go through the pros and cons of both DOT4 and 5—
DOT4 pro’s —
Well established and well known.
Cheap(er)
Has corrosion inhibitors to cope with absorbed water
Doesn’t ‘trap’ air as much as DOT5, more incompressible
Better lubricant (particularly important where ABS fitted)
DOT4 con’s -
Absorbs water into solution, generally up to 3% w/w.
Needs 2 yearly changes to ‘dry out’ and replenish corrosion inhibitors
Attacks paintwork (minor point)
Slightly lower boiling point than DOT5
Now for DOT5 —
DOT5 pro’s -
Doesn’t solution absorb water
Higher boiling point
DOT5 con’s -
Expensive
More compressible
Doesn’t mix with either water or DOT4
Still no clinching arguments, so onto more subtle ones—
DOT5 and any water (pre-existing or otherwise) Because they do not mix, any water
will settle out at system low points and cause
corrosion. Hence DOT5 is not a universal panacea on an existing system. (At
least DOT5 holds the water in solution where the corrosion inhibitors try to
neutralise it.)
Flushing DOT4 out and hoping to get DOT5 in is an almost impossible task, Dow Corning
tried this in a major test and 30% of the old DOT4 remained in the system.
However this didn’t cause any major ill-effects, although some sludging occured.
Where a full strip down was done before changing to DOT5, the full benefits were
obtained – zero wear or corrosion and only 0.01% water ingress.
The conclusion seems to be that DOT5 performs best in a new or fully stripped-down
system, but can still perform reasonably well in an older system by flushing.
I would be concerned however about the mention above of ‘some sludge’ if I had
an old system where the DOT4 hadn’t been changed for yonks then I wouldn’t
even consider doing a ‘flush-change’ the risks seem just too high compared
to the benefits.
If however I had a well maintained system, and particularly if I was removing or
replacing all the calipers etc. then I would do it.
So, you pays your money and takes your choice, and you know your own systems better
then me!
ALAN JONES