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Originally Posted by SD1too

I'm surprised to see only one person "thanking" MarinaBrian for this very helpful and relevant statement. In case anyone has missed the point, what this means is that you don't need a T4 to bleed the brakes. Expecting cries of disbelief, I offer below some basic information derived from RAVE by way of explanation.

If you look at the ABS modulator's flow diagram for any wheel, brake fluid from the master cylinder flows through the inlet solenoid valve (normally open) and thence to the caliper. 'Normally open' means that the default state of this valve, i.e. with no electric current applied to it, is that brake fluid is free to pass through it unhindered.

The outlet solenoid valves (also in the caliper circuit) are 'normally closed' so no fluid or air can pass through them unless a wheel sensor activates the ABS (which would not be the case when renewing fluid or replacing failed external pipework).

So what we effectively have is simply a length of pipe from master cylinder directly to the caliper. That's it, and this applies to all wheels. It has to be like this because the ABS is a fail-safe system. So if the ABS has not been activated there is no possibility of air entering any part which requires T4 to remove it.

I haven't found any reference to turning on the ignition activating any valves internal to the ABS modulator for testing purposes.

Simon

It could be that lack of thanks to Brian's post is because the forum consists of very many OLD folk who probably knew that already.

I can remember bleeding the brakes that way more than 50 years ago but have since got out of that habit.---