I don't think you're right there Phoenix. Sometimes air can get trapped in the abs block which needs activating to get the air out. Doing the procedure isn't diluting the fluid it is replacing it and if you used 3 litres you are doing something wrong.

How can air get trapped in the ABS block unless the system has been opened around it? - and I assure you, pressure bleeding with 'brake pedal assist' will get any air out of the system, wherever it is. ABS blocks default to open channels from the master cylinder to the outlet pipes with no control input, therefore, even on opened systems, it is a trivial matter to ensure an air-free brake hydraulic system. It's something I've been doing across a range of vehicles, JLR included, for many years.

If you think that old and new brake fluid doesn't mix as it's pushed through the pipework (and ABS block), then try this simple example, gently pour a few drops of milk into your mug of tea - and watch it disperse. Brake fluid is hygroscopic so will absorb water from the existing brake fluid as it comes into contact with it, water also affects the viscosity of the brake fluid so the new stuff being thinner will 'push' a channel through the old, leaving some in the pipes & calipers.

Normally a brake fluid refresh (often called change, wrongly) on the 2 or 3 year point (manufacturer dependent) is done with 1 litre of brake fluid, which is enough to bring the water content level down to acceptable levels. As it was clear from the colour of the brake fluid (and from a moisture content test done at the calipers, not the reservoir) that it was seriously overdue a change, I chose to push three litres through the system so that it would be predominantly fresh brake fluid coming out of the bleed nipples.

I think I'll add it to the list of jobs for Glenrands next week. I did another gravity bleed on the rears this morning but saw no air coming out but then it wouldn't would it as it would travel back up the lines to the MC. 2012 TDV8 Vogue (L322)

Phoenix wrote:

How can air get trapped in the ABS block unless the system has been opened around it? - and I assure you, pressure bleeding with 'brake pedal assist' will get any air out of the system, wherever it is. ABS blocks default to open channels from the master cylinder to the outlet pipes with no control input, therefore, even on opened systems, it is a trivial matter to ensure an air-free brake hydraulic system. It's something I've been doing across a range of vehicles, JLR included, for many years.

If you think that old and new brake fluid doesn't mix as it's pushed through the pipework (and ABS block), then try this simple example, gently pour a few drops of milk into your mug of tea - and watch it disperse. Brake fluid is hygroscopic so will absorb water from the existing brake fluid as it comes into contact with it, water also affects the viscosity of the brake fluid so the new stuff being thinner will 'push' a channel through the old, leaving some in the pipes & calipers.

Normally a brake fluid refresh (often called change, wrongly) on the 2 or 3 year point (manufacturer dependent) is done with 1 litre of brake fluid, which is enough to bring the water content level down to acceptable levels. As it was clear from the colour of the brake fluid (and from a moisture content test done at the calipers, not the reservoir) that it was seriously overdue a change, I chose to push three litres through the system so that it would be predominantly fresh brake fluid coming out of the bleed nipples.

Oh well - perchance you do read this 'gaff', tell me what you think happens in your view of fluid dynamics?

I only comment on matters I have relevant knowledge of, not just an opinion.