Originally Posted by JOHNF
David (or others)...explain how the weep hole works, and how one uses it to catch an early problem with the slave cylinder. It is so buried in the bike, how is it observed or checked?
I watched the weep hole video...great stuff, and it made the task look time consuming but basic. I have no apptitude for mechanic skills, so would be intimidated by the task. As usual those with mech skills have special tools, and design jigs to help with specific tasks (kind of like watching, This Ole House...the jigs he builds are more complicated that what I could build as a finished product!!)
Bike goes in tomorrow.
It is not used as an indicator, but a DRAIN to let the brake fluid drain out when the cylinder fails, and not get into the clutch.
I was the first one to do this to an LT, and knowing what the hole does refuse to call it a "WEEP" hole. Often when the slave fails it is pretty fast, and fluid does not just "weep" out of the cylinder, but pretty much gushes out past the failed piston seal. Without the hole drilled, there is absolutely no where for the fluid to go but down the actuation rod bore in the transmission input shaft and right into the clutch. The fluid is actually PUMPED into the clutch by the action of the piston moving back and forth as you pull the clutch lever in and release it. The space in front of the cylinder is SEALED to the back of the transmission, and even has a gasket there to insure it is sealed. A very poor and unfortunate design by BMW. So, this is not a nice, slow, weeping of fluid, but pumped slugs of fluid. Therefore, I drilled a DRAIN hole to allow it to go out through the hole easily, before it gets into the clutch.
The fluid has to get high enough in the mount bore to reach the area of the hole in the shaft where the actuation rod passes through, so with the hole in place, the fluid will likely drain
out before it reaches this level.
Even with this hole in place you will not likely ever see outside evidence of the failed cylinder before you know it has failed completely. Remember, there is not all that much fluid in the clutch circuit in the first place. By the time any of us knew the cylinder had failed, pretty much all the fluid that was in the clutch circuit had disappeared, either into the clutch, or out the drain hole if one was lucky enough to have drilled it before hand.
Everyone who insists in calling this a "weep" hole just does not understand what is going on here at all. Weep holes have been used for decades in automotive and industrial areas to allow SMALL leaks to get out, and in many cases indicate that there is a looming problem. That is NOT the failure mode here.
I know that one of my cylinder failures was caused by the little "throwout" bearing in the end of the slave cylinder piston tightening or freezing, causing the piston to spin in the cylinder, which wipes it out practically instantly. I posted pictures of the piston and cylinder bore once, very evident that the piston spun, causing heavy scoring of the cylinder, then pretty instant failure of the piston cup seal. It is my opinion that many of the failures are caused by the failure of the tiny ball thrust bearing, which is spinning all the time the engine is running. Again, a very poor design.