Volume of brake fluid in the lines?

[rotorwings]

Can anyone tell me roughly what volume of brake fluid would need to be flushed through the lines to be sure the line is completely flushed? Obviously it is going to be slightly different for each corner.

I am changing the fluid and I'm not confident I will be able to see the difference in colour from the old fluid and the new fluid.

Edited by rotorwings on Tuesday 5th July 09:32

[fergus]

assuming that the internal diameter of a 4.5mm solid brake line is 2.5mm, and assuming you have 4 x 2m runs from the master cylinder, then this equates to about 40 cm3 of fluid. You also have the fluid in the caliper behind the pistons, and the master cylinder to consider. I reckon flushing through 100 cm3 should have new fluid throughout. Try a few drops of blue food dye?

[spanky3]

water-based dye in brake lines.. is that wise?

[Farlig]

By at least a half litre & just use up the whole bottle... it doesn´t have a shelf life once opened anyway - well it´s better not to keep it at least... Then you know reservoir, lines and all calipers thoroughly flushed

[Dr Slotter]

Farlig said:

By at least a half litre & just use up the whole bottle... it doesn´t have a shelf life once opened anyway - well it´s better not to keep it at least... Then you know reservoir, lines and all calipers thoroughly flushed

This.

[rotorwings]

Good point on how to calculate the actual volume. Maths is magic!

Also a good point on using a full 500ml bottle. I thought I might need a litre, so it is good to know that using a 500ml bottle will definitely cover it (according to the maths and general advice). I already have a fresh 500ml bottle here, so that's perfect.

Thanks all.

[mharris]

Maths is good, but my unscientific advice is that there is a lot less volume in the pipes than you imagine. Even filling from empty you get fluid appearing at the caliper incredibly quickly.

If you want to do a thorough job then you can wind-back the rear calipers. This will reduce the volume in the cylinders that needs to be flushed. It is a lot more faff though and is really only useful when filling from empty to prevent bubbles getting trapped.

[fergus]

spanky3 said:

water-based dye in brake lines.. is that wise?

two or three drops in 500ml will change the colour, but will have a negligible effect on the wet boiling point of fresh fluid.

look at the diff between the wet and dry BP of most fluids, then check what the test is to determine "wet". A few drops of food dye won't turn dry (fresh) fluid into a "wet" state.

[HAL2016]

When I worked in the trade as a fitter we used a clear bottle and tube to check the change of fluid colour..when it ran clear you were done..I never heard of anyone putting any sort of dye in the system and wouldn't recommend it myself.

[HAL2016]

When I worked in the trade as a fitter we used a clear bottle and tube to check the change of fluid colour..when it ran clear you were done..I never heard of anyone putting any sort of dye in the system and wouldn't recommend it myself.

[HAL2016]

When I worked in the trade as a fitter we used a clear bottle and tube to check the change of fluid colour..when it ran clear you were done..I never heard of anyone putting any sort of dye in the system and wouldn't recommend it myself.