[RaYzerman]

To my knowledge, there is no check valve, nor would you want one. When you release the brakes, fluid needs to flow freely back to the reservoir to relieve pressure at the calipers. Take a look at the Gen2 ABS flow diagram below.

Also, with ABS not activated, the fluid is in by-pass of the ABS valves for normal braking. Only when ABS is activated that is shut off and fluid directed into the ABS fluid circuit within the ABS hydraulic unit, which pulses the pressure/relief sensing wheel lock-up. There is fluid in all the lines to and from the reservoir to ABS unit to the calipers, should be no air. Rear circuit separate from front, of course.

The master should be intaking fluid into the piston which then pushes it to the calipers..... then there is a return hole also beside the intake hole in the reservoir. Since everything is immersed in fluid, it would seem the master is not pushing fluid, thus lever going to the bar. I'm not seeing why there would be any other reason.

I am not sure why you had the same symptom with the '07 master, other than air trapped in the master.... to fix that, crack open the banjo bolt at the master and lightly press the lever, air will escape past the banjo washer and you should then have the master bled and working. Snug up the banjo bolt. Of course, put an alcohol soaked rag to catch the fluid, and clean up with isopropanol. If you have pressure and the lever creeps back to the bar, that would suggest you need to rebuild the master with fresh rubber parts.....

The ABS test procedure just flushes out the ABS fluid circuit by pushing a small amount of newer fluid in.... after activating, I'd recommend a quick squirt bleed, then repeat the test.... and repeat this perhaps 3 times.. this will ensure no stale fluid in the ABS fluid circuit, a common issue with this generation and prior of ABS systems on FJR's. Although again, I've not heard of a front ABS pump issue, more likely happens on the rear. I don't think you have an ABS pump issue on the front at least.

 

[RaYzerman]

There is a service who rebuilds the ABS hydraulic units, FYI. Thankfully. But you're not there yet.

 

[RaYzerman]

In the flow diagram, Item 5 only opens to let fluid into the ABS fluid circuit (5,6,7) when ABS activated, then likely it acts as a check valve to not let it flow back out unless electronically it opens as ABS pulses. Doesn't make sense to me, given the master is providing constant pressure. The fluid would flow to the solenoid valve item 7... bottom line, all fluid circuits are full no matter what, and if bled, there should be no air in there. When ABS is not activated, it bypasses 5,6, and 7 for normal braking. When you are bleeding or using your brakes and get a spongy master cylinder, my opinion is it has nothing to do with the ABS hydraulic unit. I'm back to the master cylinder somehow and ensuring it is primed with fluid..... however, how to test that. If you were somehow to plug the banjo fitting at the master, can you build pressure... then try the same thing on one of the front calipers..... if OK, then try the opposite one.... if all that was OK, then there's no issue with the ABS hydraulic unit since all fluid passed through there first.......

So the next question is.... do we have a stuck piston in one of the calipers which is trying to deploy and being delayed (lever losing pressure), and retracting more than normal (potentially due to bad caliper seal?).
Another test you could do is plug the outgoing line on the hydraulic unit leading back to the front, but that should have been ruled out already if the above was successful.

It is possible to have air trapped in the hydraulic unit, so when you bleed, open whatever bleeder up front and squeeze the lever quick and hard to the bar, then quickly release it. Repeat a couple of times..... not fixed, then I suppose the only way to get the air out is to loosen the fittings at the hydraulic unit and let air escape .... that gets a bit messy, so have lots of rags and isopropanol to clean up....

 

[RaYzerman]

I swapped one out on a '07 at a tech day, took it home.... yeah, those "rivets" are a special thing (one can get replacements for), but screw it, leave it to the professionals....
Rebuilder here:

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[RaYzerman]

The only thing we thought in the past was blockage was from old dried up brake fluid purged itself into a line downstream of the ABS valving..... one could try to soak/flush all the passages with a solvent that would dissolve brake fluid.... almost any naptha/alcohol would work mildly without hurting anything, at least while it's all hooked up..... you could go acetone or toluene later.... I wouldn't think a lot of rubber parts inside, although if still hooked up, I'd be concerned about the rubber (or whatever they are) brake hoses.