Hmmm you're very trusting,
Really to take this further, do I NEED to do an oil change, brake fluid change at all - may I please have an answer that explains it in engineering terms, rather than people just do it etc etc.....
Should I just ride it for a 1000 miles or so.
You asked for it
DOT = Dept of Transport (in the good ole’ US of A). The higher the number following DOT then the higher the specification (or boiling point), and ranges from DOT 2 – DOT5.1. (Please note DOT 5 spec fluid is silicone based – more info on this shortly!) If you look at your brake fluid reservoir on the top or nearby you’ll see, “Use DOT3 fluid only.” That will probably be on an older bike (like an 80’s model). What it’s actually saying is that the minimum specification for brake fluid in this system should be DOT3, but in reality you may in this instance top up the fluid with DOT3 or 4, or change the fluid for DOT4 or DOT5.1. Note that is DOT five point one, each of these are higher specification than the original DOT3 fluid.
Now all of these brake fluids are what’s collectively known as Polyethylene Glycol esters, and basically they are made from a substance very like concentrated antifreeze! Perhaps that is a clue to why mechanics (or should I call us technicians these days?) and manufacturers request that brake fluid should be changed every 2 years (even earlier in servo ABS systems). No? Still not enough of a clue? Well, read on, for the fact is that antifreeze mixes very well with water – that’s its purpose – and for the brake fluid to replenish the space that’s left as your pads wear down there must be an allowance for air to enter. And if air can get in, then so too can water, (carried as vapour in the air). Water can also enter the system by a process known as diffusion, where water ‘creeps’ in through rubber seals and hoses (this happens over a long time).
As the amount of absorbed water increases over a time period your clean and fresh, golden coloured brake fluid will turn to a brown, contaminated liquid (the darker the colour the more contaminated the fluid has become!) that’s not very good at dealing with the high temperatures created whilst braking. The following table illustrates the difference in efficiency shown for just 3% of water absorbed (ie the wet boiling point):
..............................Dry Boiling Point ..............Wet Boiling Point (i.e. 3% water)
DOT 3 ..........................205ºC..............................140ºC
DOT 4 ..........................230ºC..............................155ºC
DOT 5 ..........................260ºC..............................180ºC
DOT 5.1..........................270ºC..............................190ºC
The brake fluid also develops slightly corrosive tendencies and your system’s seals will degrade! So folks now you understand why we ask that you change fluids etc every 2 years – it’s not because we get more money from you, but instead it’s just down to the simple fact that the fluid can’t do it’s job properly anymore!
Now back to DOT 5 spec. This fluid is a silicone based brake fluid and under no circumstances should it be used in a standard DOT 2, 3, 4 or 5.1 (five point one) system, or even mixed with any of these brake fluids. Indeed, an agreement was reached that silicone brake fluid be coloured purple to help prevent end users mixing it with standard non silicone brake fluids. Despite this, a few people might still be thinking, “That’s great! If I use silicone fluid in my bike it can’t absorb water and I’ll have good brakes!” Wrong, I’m afraid, for the following reasons.
Silicone based brake fluid is only half as resistant to compression as polyethylene gycol, hence you will experience longer lever or pedal travel to gain the same brake effort. In other words your brakes will be spongy – all of the time! Another note – some Harleys use this stuff as standard; need I say more?
Silicone brake fluid will not absorb water under any circumstance; the water therefore must collect at the lowest point as it is more dense than the silicone, and the lowest point is your caliper. Water boils at 100ºC, but the temperatures that can be created by a long, drawn out braking manoeuvre will exceed that quite dramatically, so turning the water to steam and further reducing your braking capabilities – in fact they will probably not work at all!
To sum up simply, never, ever use silicone brake fluid in a braking system that it is not specified for. The silicone could attack the standard seals used in a regular polyethylene glycol system – but if you have done this, well at least the stuff’s purple so you will see it more easily when it does start to leak!
