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Brake fluid condition and mot
corrado vr6 - 10/7/14 at 08:02 PM

Hi there,

My question is does brake fluid condition get checked at an mot?

I have just bought a brake fluid tester for use in the garage, I checked my tintop which the brake fluid has recently been changed, the tester I bought said it was ok

I checked my girlfriends car recently just passed mot after issues with the brakes and my tester shows dangerous (she has just informed me it has never been serviced in the time she has owned it) would the mot garage have not checked the condition of the fluid? Or is it a case as long as it's there it's a pass?

Got me thinking

Cheers Greg

adithorp - 10/7/14 at 08:06 PM

No.

Smoking Frog - 10/7/14 at 08:11 PM

If it passes the braking test no questions or checks would be made of the fluid. This is just based on my experience when I've stay with the car during the MOT (last Friday as it happens).

rusty nuts - 10/7/14 at 08:24 PM

Brake fluid level should be checked at an MOT test and a visual check is made for any leaks , no tests are made for fluid condition . Many people believe that an MOT test ensures that the vehicle is in good condition when in fact it only ensures the vehicle meets minimum requirements, things like ball joints have to be almost to the point of breaking to be a fail

corrado vr6 - 10/7/14 at 08:53 PM

Ah ok that clears that up then, thanks all

Now to change the fluid

trikerneil - 11/7/14 at 05:16 AM

What's not OK about the brake fluid?

How does brake fluid "go off" in a sealed system?

Neil

corrado vr6 - 11/7/14 at 05:26 AM

This is my understanding
After time it degrades and can get moisture in the system, moisture is compressable therefore spongy brakes which aren't as efficient

cliftyhanger - 11/7/14 at 06:29 AM

Brake fluid is hydroscopic, basically it is very absorbent of water.
This causes the boiling temp of the fluid to drop dramatically, if the fluid boils you will have no brakes.

Something people just do not think about. But really ought to.

Fluid changes at a quick fit type garage can be cheap, me dad was offered one for £25 when he bought some tyres.....

mcerd1 - 11/7/14 at 08:20 AM

quote:
Originally posted by cliftyhanger
Brake fluid is hydroscopic, basically it is very absorbent of water.
This causes the boiling temp of the fluid to drop dramatically, if the fluid boils you will have no brakes.

^^ what he said

dot 4 and 5.1 fluids have very high boiling points (230°C and 260°C minimum) but water is only ~100°C (at 1bar)
with 3.7% water in them the boiling points are down to 155°C and 180°C min. - more water reduces this even further....
if it ever boils then its turning into gas which means its turned from an incompressible liquid to a compressible gas - so you pedal suddenly feels spongy and the brakes don't do very much

the system isn't as sealed as you'd think, tiny amounts of water will slowly get passed the seals and the reservoir actually has a small breather hole somewhere to allow the change in level as you press the pedal

any moisture in the air is absorbed by the brake fluid - that's why its important to keep any spare fluid in sealed bottle (preferably an unopened one)
this hydroscopic property is actually a safety feature - it means any moisture in the system gets absorbed and distributed through the fluid so small amount won't have much effect on the overall boiling point and will get flushed out when the fluid is changed

DOT5 silicon fluid is not hydroscopic and is naturally a bit more compressible as a liquid (so the pedal never feels as solid)
but this is minor issue compared to what happens when very small amounts of water get into the system - instead of being absorbed the sink to the lowest points and stay there, that's often the calipers so you've got small bubbles of low boiling point fluid at the hottest part of the system and the water can allow the pistons to corrode faster and size the calipers - neatehr of which is good for anyone...

Slimy38 - 11/7/14 at 09:46 AM

The easiest way to tell is compare the colour of your existing fluid with some new fluid, if it's a few years old then the colour will be noticeably different. You use that colour change when you're doing a fluid change so you know you've got good stuff all the way through the lines.