Time to post a little question for everyone...

My 4x4 1992 DOHC Saff has just started with an intriguing ABS issue. About 2 weeks ago, I had both red and yellow dash lights coming on in the dash. Checked the reservoir and the fluid level had dropped about 1 inch max. Went looking and found that the passenger rear caliper bleed nipple had managed to wind undone slightly, just enough to allow a little bit of fluid to weep when the brakes were pressed, but not enough to drop any fluid on the wheel.

Tightened that up and refilled the reservoir accordingly and immediately lost the red warning light (no surprise). But, I still had a "weird" brake pedal.

So, last weekend I put a new set of rear pads in (they were wearing down anyway) and bled both rear brakes out. Fine. But, I still randomly get a weird brake pedal and yellow ABS light coming on.

An example: start the car and the ABS light does its self-test before going out in under 20 seconds. Drive down the road and it will drive fine until a random moment when the brake pedal starts to pulse as if either the ABS is engaging or the pedal is pumping up. The next time the car gets to 20mph or above, the ABS light comes on as if the system has detected an ABS sensor fault (even though it has passed its own self-test earlier in the same drive period). The light will stay on until you turn the ignition off. You can restart the car immediately and the problem will go away until another random moment.

At no time does the car attempt to lock-up under braking, or anything similar.

The fluid level is good, all the connections to the sensors, pump, reservoir cap, etc... are good, clean and still coated with their protective grease.

I have not performed an ohms check on the sensors yet (will do this over the weekend when I have time). The fluid is DOT 5.1 as it has been running for over 7 years and is bled out every time I change front or rear pads (normally every couple of years or 15k, whichever comes first).

I have spare ABS valve blocks, reservoir caps, ECU, etc.. but am not sure on sensors themselves.

The wheel bearings seem okay and the original dust covers are over all four driveshafts to protect from dust ingress.

Thoughts on this one?