Intermittent brake fault - puzzled and confused

A couple of weeks ago, I posted here the story of my ABS/brake failure on a 2005 reg 1200GS. At the time, the general agreement seemed to be a sticky valve on the ABS pump.
It's not over yet.
Today I finally got round to taking the Goose to my mechanic. While I was away for work last week, the lovely guys here at UKGSer suggested a way to fix the fault for £250, not £2000, by cleaning the valve. Thank you very much, Steptoe.
But - surprise, surprise - when I started the bike this morning, the fault was gone!
15 miles later, though, it came back.
So I stopped for a coffee. When I returned to the bike, not only was the fault there again, but also the speedo wasn't working.
Saddened and bemused, I carried on along my way to the garage. Got there, switched the bike off, chatted to the mechanic, started the bike up again - no fault. The computer says "Rear brake pressure too high - this fault is no longer present".
Rode around for about half an hour - still no fault. And the speedo is working perfectly again.
Very confused now.

The speedo sensor is also the ABS rear wheel speed sensor. It's fitted inside the final drive. Because it is magnetic it attracts microscopic wear particles. When coated with them, it stops working so you get dodgy speedo indication and ABS faults. Try cleaning the sensor, if you search on the forum you should find info on what to do. I could be spouting bollox as usual but you've nothing to lose and cleaning the sensor is free and it's easy to do.

Thanks - will give it a go.

Look what I found on advrider - sounds even scarier/dearer!
"Just in case you did not understand the highly criptic responses. One of the symptoms of a failing final drive is metal on the speed sensor. This will screw up the speedo. Yours goes to 0. When mine went, it headed to 150 MPH at a stop sign then slowed to 0 going down the road. You don't need to take the bike to the dealer just yet. Take off the rear wheel. Look through the brake disk. Do you see that wire that goes to the inside of the upper part of the final drive? Thats the speed sensor. Unscrew the thing that holds it in and examine the inside. If its covered with metal flakes, someone must have forgotten to change the FD oil and its well on the way to a failure".

Sensors are magnetic, get it checked by a person used to seeing final drive faults, you could check sensor for metal particles, save pieces & take to get checked.
There is someone on here who overhauls the final drive assemblies, I've seen his posts & after looking at the tools & procedures he looks very professional, much cheaper than an abs replacement.

r1200gs final drive rebuild.
£350 all in .this is unit only ,plus p&p,more info email overlandandclassic@hotmail.co.uk

Mikeyboy is his name

Thanks!

Check out the scary final drive threads posted by Patzx12
http://www.ukgser.com/forums/showthread.php?166875-The-inside-of-my-FD&highlight=final+drive