How much fluid to flush NC brake system

Technical Area Wheels, Tyres & Brakes
2

Hi All,

 

How much brake fluid do i need to flush the brake system on my NC?

 

Thanks!

3

I may use a bit much but I would use about 1.5 litres on a MK3.

4

I’ve flushed hundreds of brake systems and 1litre will be plenty. Use a syringe and a bit of thin hose, or similar, to draw as much fluid as possible from the reservoir then top it up. Then pump fluid through each bleed nipple with a length of clear tube going to a bottle to collect the old fluid. I start at the front giving about 20 pumps for each front wheel and about 10 for each rear. Let the reservoir get very low but make sure you don’t let it run dry or you’ll have to bleed the air out of the system. If your brakes are badly contaminated and/or the fluid is very old then you may need a bit more, but I rarely do.

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5

Nick are you saying here you have changed Mk3 fluid and you only used 1 litre to get new clean fluid out of each nipple or on a point of principle you will not use more than 1 litre due to the cost.

Not saying I’m right here but it seems that you use less fluid to do the rear brakes when they are further away from the master cylinder than the front ones are and the brake poportioning valve tends to reduce the amount of fluid coming from the rear.

Also what has the age of the fluid got to do with the amount you change. If you are replaceing all the fluid because of possible water absorbtion ( that is the reason for bleeding the brakes) then you are replaceing all the fluid, therefore your 1 litre used would not change at all depending on the fluid age.

6

Tha actual amount of fluid in the system really isn’t that great. If you’re doing a regular two year fluid change then the system isn’t going to be badly contaminated and if you’re starting with a virtually empty reservoir it doesn’t take much to flush the old fluid out. If the system is badly contaminated then it’s likely to take a bit more.

The front calipers are larger and pumping more fluid through them first cleans out both sides of the master cylinder, so you don’t need to pump as much fluid through the rears.

If you want to completely change the fluid without any contamination then you’d need to drain the system completely and refil, but generally that’s not necessary.

 

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7

I’ve done six sets of car brakes this year so far (also a few years back designed and built a complete system for my kit car, converting it from drums at the front to discs).

The main reason for changing brake fluid is because it gradually absorbs water vapour from the air. As well as the potential to cause corrosion, “wet” brake fluid has a lower boiling point than clean fluid. The problem is that if the water in the brake fluid boils out, it produces vapour which can displace the fluid back up the pipework; pressing the brake pedal may not then apply sufficient braking force to the pads.

For that reason, I prefer to flush the entire system very thoroughly. I use a clean container to catch the old fluid (I normally use a white plastic “Eazibleed” bottle) and monitor the colour of the fluid as it’s bled out. When it matches the colour of fresh fluid, I know the job’s been done. Trouble is, the fluid might look only slightly discoloured but still contain water. So I probably waste a little fluid for peace of mind.

I’ve often wondered why manufacturers don’t more widely offer a choice of two very different colours of otherwise identical brake fluid (I’ve only ever been offered pale yellow). The colour of fluid used could be alternated each time the system is bled, making it very obvious when the system is fully flushed.

Forgot to mention, I use a cheap turkey baster (from Wilko’s kitchen dept.) to remove the majority of the old fluid from the master cylinder.

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