Low/free brake pedal (WJ)

[Domaso]

My WJ (2.7 CRD if it matters) has a "low/free" brake pedal - about 70% of pedal travel yields almost no braking power, and only the last 30% of travel is breaking. Pedal returns to it's original position when released, all other pedal/braking behavior seems normal.

First I thought I might have air in the system - ran through 2l of new brake fluid when bleeding to be extra sure. Also replaced all 4 rotors, all brake pads, and made sure the caliper pistons and sliders are not stuck - all good, but still the problem persists.

What should be my next suspect? Master cylinder, Brake booster, ABS module? How can I check those? (I don't want to start throwing parts at it, so I'd like to do some reliable checks). As far as I know, brake booster pushrod is not adjustable on WJ so I ruled that out.

I found one test for master cylinder (below) - seems to be reliable test, but it requires brake line removal - any easier ways to test it out?
Btw, anybody knows if a certain size bolts can be used to plug brake line connections or do I have to buy specialized plugs?

Test the master cylinder. Open the master cylinder reservoir cap and remove the brake fluid from the cylinder using a syringe or baster. Carefully remove the brake lines from the bottom of the master cylinder and securely plug the line connections on the master cylinder with suitable caps or plugs. Refill the reservoir to the appropriate level, replace the reservoir cap, start the car and apply the brakes. If the brake pedal now feels firm and high, the master cylinder is functioning properly.

[Classic Boy]

You should always bleed the master cylinder Before bleeding the system if you believe there is trapped air.
Also make sure the calipers are not swapped LH to RH and the bleeders are no longer uppermost as if they are wrong sided you will not get the air out completely.

[JeanLuke]

There is a procedure to bleed the ABS module that includes putting it into a special bleed mode. Have you done this?

[bodgie]

What procedure did you use to bleed the system? Depending on how you did it you may have actually made things worse. How I know this is because I've been there and had a similar issue in the past.

Here's the correct manual bleeding procedure, my suggestion is to make sure you do step 2 again if you haven't already done so. Just let gravity do it's thing for a bit an d this should help a lot.