Master Cylinder - hole sizes between the resrvior and cylindrer

I will check tomorrow but from memory the small one was 1mm and the large one 3mm when I added an alloy sleeve and needed to drill through to the new piston chamber.

Yes, they are 1mm and 3mm with the 1mm at the center, when you fit the piston the seal must draw back past the 1mm hole allowing the pressure that has built up to release back into the reservoir. If you do not get this part right then in hot temps your brake will stay on and not release and bleeding is made worse. Adjust the length of the piston until the seal pulls past the hole completely.

For the avoidance of doubt (as the lawyers put it) can John confirm that this phenomenon is due to differential expansion causing a change in the position of the piston relative to the cylinder?And, likewise, that it is a result of ambient temperature rather than heat generated by use of the brakes?My specific reason for checking is that the disc brake on my recently-acquired Fastback needs the usual gorilla grip to get any serious braking effort, but then shows signs of not releasing -- even in what I would call cool weather.Previously john_holmes wrote:

... If you do not get this part right then in hot temps your brake will stay on and not release ...

I have never experienced the issue but I have seen it mentioned a lot in the US in the hotter States so it is related to ambient temps as it mainly seems to happen after the bike has been in the sun after a ride. However if the piston seal is only just giving an opening then it would not take much for it to then not give an opening at all. The check would be the next time it locks on then open the bleed nipple and the pressure should result in a spurt of fluid being ejected and then after the nipple is closed the lever should then be moving again until the pressure builds again.

Be very wary of shortening the lever where it engages with the end of the piston, if you take too much off the end can over travel and go over the end of the piston and lock in that position, resulting with brakes on and no way to release. So only take material from the end of the piston. In fact you should check for this condition at the same time.

For brake pads not releasing also check your hydraulic piping to the caliper. If the pipes are old they are most likely gunged up and will not allow proper flow of fluid back into the master cyclinder / reservoir when brake lever is released , causing pads to only partially release from disc.

Had this problem on fitting a Norvil front brake upgrade kit after having no luck improving brake action despite fitting all new parts to original Norton caliper and master cylinder on my MK 1a 850. Stiff lever action plus pads staying partially locked on remained exactly the same on the new upgrade set up until - hey presto - changed the hydraulic piping for new (flexible , one piece pipe from master cylinder to caliper) and brake action was then perfect with light action and instant release of pads.

I had this problem on my home made hydraulic 2LS rear brake. When I upgraded from a Triumph herald master cylinder to a Honda CBR600 item, the piston wasn't coming back far enough. this meant that the fluid was trapped and couldn't expand back into the reservoir. that caused the brake to come on as the fluid expanded. It's amazing how that heats the drum and sprocket to smoking levels. Some of you might have seen me dealing with it in Italy. A simple adjustment of the piston sorted it.

I have fitted a sleeve but not the RGM one, I can't see how it can work without extending the holes through the sleeve into the piston chamber.

Previously john_holmes wrote:

I have fitted a sleeve but not the RGM one, I can't see how it can work without extending the holes through the sleeve into the piston chamber.

john

shine a torch into your new sleeved master cylinder, you should be able to see two drilled holes ,they may not be in the twelve'o clock position as on original,There should also be two grooves turned about the o/d of your new insert sleeve,which should match -up with your original master cylinder drilling

hit it with an air line to conform that you have flow from the master cylinder to the brake fluid reservoir,

I would not just drill through the original holes in the brake fluid reservoir, as it will leave burrs and destroy your brake piston seals

steve

I have had this problem on several old cars/trucks ,both times when in the Alps, sometimes caused by seal swelling and extra heat. Also had total brake failure decending the Alps!, cured by the desperate ploy of pouring beer over the calipers!. The main cause was rusty/pitted discs unable to conduct heat away from pads.New fluid every year and engaging low gears made no difference ,4 tons takes some stopping.

My sleeve was alloy and had no holes, I deburred them in the bore after drilling which as it was alloy presented no problems. If the the holes are in the sleeve before the sleeve if fixed in place then its unlikely the holes will line up with the master cylinder holes but as long as there is clearance on the OD there is a route for the fluid. However instead of being able to see the seal move back past the hole to confirm the route is clear you will have to blow through the fluid outlet and check that air can get to the master cylinder reserve chamber when the piston is in the off position. Problem being as you cannot see the seal then you could have it only partially past the hole allowing a route which then heat expansion could block off the hole later.