Well, on Monday I decided to change the brake fluid on my Moto Guzzi V50. It's now 4 or 5 years since I changed the fluid - when I changed the 20 year old flexible brake hoses. (I like my brakes to be "nearly new" - as they may one day need to save my life from my - or someone's -inneptitude). The bike is 44 years old now, and the single disc front brake has had a stuck piston once before. So (stupidly!) I drained all the fluid from front and front-rear combined circuits, then dismantled the front caliper that had a seized piston. Wrong... I needed a fully bled system to hydraullically release the seized piston. So I filled the system with new brake fluid - it has a common reservoir for front single and the Front and Rear combined circuits. Then (as many Guzzi owners know) I tried to get the new fluid to fill and pump through to calipers for bleeding the air out of the systems..... The Service manual suggests "after bleeding, run the bike for a few trips after which more air will have accumulated so it can be bled..." or words to that effect! - it actually says "the brakes will improve as the remaining air finds its way out of the system"... Believe that? I don't. And a parts supplier suggests you buy some special bleed screws ... "Guzzi brake systems are notoriously tricky to bleed, this special bleed nipple can save you hours of tedious squeezing and pumping!"
- So after re-filling the system I spent hours "squeezing and pumping" the air and frothy brake fuid and air from both front and combined circuits. I was sure I could not get any more air from any of the 3 calipers.
Then I managed to un-seize the front caliper piston. I thought there was some rust on a piston, having removed the outer dust seal, so ordered a service kit of seals and new pistons for the caliper. After which I tried the bike - slowly - on the road, and found I had barely any front brake - it went from "hard to spongy" in 3 or 4 tries on the road, and then the combined system (foot pedal) did the same...
So on Tuesday, I re-bled the brakes... After "hours of tedious squeezing and pumping" I felt the brakes were much harder, so tried on the road and brakes were reasonably good, if not at their best.
So on Wednesday, when the new service kit of seals and new pistons arrived, I serviced the recalcitrant front caliper, and after "hours of tedious squeezing and pumping" I had bled more air from both circuits. So now have good hard feeling brakes.
But this is what I found in the dismantled front caliper - despite the fuid in reservoir and pipes being a lighter colour, and clean... it is not stewed-tea - just rusty and oxidised brake fluid. The fluid from the bleed nipple was the clean new fluid I had just introduced! Obviously, this old stuff can never be flushed-out.
The dark and rusty fluid is a mix from both cylinders of the brake with a seized piston. One (Seized) piston was very rusty, the other not rusty, just slightly tarnished (wiped clean with a good rub from a cloth).
Next time I change the fluid, I'll service the calipers first...
So My suggestion to all is that on your "OLDER" vehicle, (out of warranty!) you dismantle the calipers and remove the old fluid (it holds dissolved water that rusts the pistons) that does not get changed when you (or your garage) does the 2~3 year service brake fluid change.
The hazzard to health of not doing this is a seized caliper - and the brake is much less effective when you really need it most to avoid a bad crash. ANother possible risk is that the boiling point of the fluid is so reduced (normal new fluid boils at 330deg.C or thereabouts) and can get hot enough during braking (e.g. emergency stop from max highway speed fully laden with family, luggage and Mother-in-law...) so the water boils (under pressure!) and the steam bubbles cause a loss of braking (the steam forces the fuid back up the pipework so the ABS cannot work properly). Racers call this "brake fade" - and it means you cannot stop in the shortest distance you should be capable of stopping... so you hit something you didn't plan to hit. - Just before the Mother-in-law hits you for driving badly!
Not exactly Model Engineering, but I hope it helps someone?
I now have to order 2 more service kits and clean the calipers of the remaining 2 brakes that will still be full of "Old Wet fluid"! - but that must be after Easter!
Happy weekend everyone...
K2