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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: John Lang on September 27, 2016, 04:57:38 PM
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I am about to do it on my '87 K75 (non-ABS, drum rear brake) having read various 'how-tos' and sensing that perhaps the front brake lever needs more pull than it used to. But I question why bleeding is ever necessary? If there is air in the line would it not find its way to the reservoir, the highest point? Perhaps my (original) brake hoses are stretchy and need replacement. They look OK. No big deal, but would the need to bleed not be a sign of bad design? Is brake bleeding like applying leeches, an old wives' tale?
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if your brake lines are original....Replace them!!!!!!!!!! NOW!!!
yes, you must bleed the air out of the system. no, it does not all flow up to the resevoir. no, that is not a sign of bad design.
actually, sounds like you don't just need to bleed the air out....you need to flush the old brake fluid out. it must be done regularly. when you replace the lines, you will end up flushing the old fluid out and will see why bleeding the air out is needed.
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Thanks mlytle. The brakes were attended to a year ago by the local BMW dealer. I merely wanted to do it myself this year when the question occurred to me -- is this really necessary? Why does my car run for years without a brake fluid bleed? I know about hygroscopicity (a word?). I am not averse to maintenance, in fact I am keen to do it myself as much as I a can. It seems to me that a brake design that does not get air bubbles back up to to the reservoir is not very well engineered. You are no doubt correct about my brake hoses. I'll change them.
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Actually, I've had good luck getting stubborn air bubbles out of the front brake system by parking the bike nosed uphill, found that out completely by accident but it firmed up the brake wonderfully. Good question why cars seem to go forever without fluid changes, but after pricing new master cylinders I'll flush mine every year or 2.
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Good question why cars seem to go forever without fluid changes, but after pricing new master cylinders I'll flush mine every year or 2.
I also flush my motorcycle brake systems every two years, it's cheap insurance against more expensive repairs. Because it is hydroscopic the moisture content will increase over time and cause all kinds of problems.
Working in an auto dealership I see the results of no fluid changes. No longer is it just a stuck brake piston or brake cylinder it now affects the ABS units and costs big bucks to replace.
I use a MityVac or other other fluid evacuator to draw the fluid out of the master cylinder and refilling it with fresh fluid and then use it at the bleeders to draw out the remainder.
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cars don't last long times between fluid changes...people just ignore the required maintenance. every car service manual i have seen says fluid flush every two years. as nick said, i too have seen the results of ignoring brake maint in cars. it isn't pretty. it is more critical to do on bikes because, well, they are bikes and the consequences of brake failure are more severe.
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Brake fluid absorbs moisture, 1-2% a year. With older bikes (and cars) where this hasn't been done this can be an issue for a number of reasons.
When I do brake work on cars I NEVER push old fluid back into the system. I always crack the bleeders open to push cylinders and calipers back together. I suck out the reservoir, replace with new fluid (repeat if fluid was really dark), do the brake work, then bleed the system at all 4 wheels until fluid is clean/fresh.
I'd do the same type of procedure for the brick
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The capacity of the system also factors into this as well. Cars and trucks have larger thermal capacity than bikes therefore the fluid gets hotter. This dynamic will introduce more moisture and lower the boiling point of the fluid. For the relatively small cost of fluid and time why take the risk of a less than optimum brake system ?
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I suppose on cars etc, there is the backup of a split system. Can we generalise and say that the average car owner is less mechanically inclined than a bike owner? If so, there will be more filthy dark brake fluid in cars than bikes. Anyway, the front master cylinder is invariably in the line of sight.
Add to this, the front brake is doing about 75% of the braking. If it goes west it potentially has significant consequences. But us aging K bike owners don't ride on the edge of course.
But if you're being a tight arse and don't really know the condition of the lines (now nearly 30 years old), the local brake shop should be able to help. I got a top front line replaced (master cylinder to steering head) ... approx $100 through BMW dealer... for $30. Take the old one(s) in and get it copied.
However, I'm being a hypocrite: the rest of the lines on mine are all old and should be replaced. The master cylinder was seized, top hose was blocked with brown crud and beyond redemption, the rest were cleared using tie wire - though I was working from the bottom up and kept progressing until I got fluid coming out of the bleeder nipple on the caliper.
Bleed yer bloody brakes. 5 - 10 mins
Guy
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I am going to speculate here that cars can go longer between bleeds because the master cylinders and calipers are cast iron vs. the diecast pot metal used in motorcycle brake components. The cast iron is significantly more resistant to corrosion caused by the water the brake fluid absorbs.
In the short time I have been riding bricks I have seen several rear master cylinders with enough corrosion to cause severe leaking and loss of braking power. Never had a problem with a car, even with the one I own now which is only 1 year newer than 2 of my bricks one of which has had it's rear master cylinder replaced.
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I had to have my rear master cyl replaced just after I bought my K11 with 62,xxx miles, even though the PO was meticulous with maintenance. Parts just wear out, I guess.