jfheath
John Heath
Sorry - I replied without reading the earlier posts.
All master cylinders are a one way valve. Fluid from the brake side of the brake line cannot get past the seal. But whent he piston is returning, fluid from behind the seal can get past. Have you ever wondered why when you fill the rear brake system with fluid, it comes out through the outer piston bleed valve just by pressing the pedal ? It is flowing past that primary seal in the SMC - easy as anything. Very much like a bicycle pump pushes air into the tyre, but easily apulls bag up the tube allowing more air to get in front of that 'leather' washer.
There are a number of ways in which the SMC can fail. All of them are to do with the SMC not allowing fluid under pressure to return to the reservoir. The only way that it can get back to the reservoir is through the tiny compensation port. In the SMC, there are two tiny compensation ports - and the fluid has to get through both of them. One is in the cylinder wall. The other is in that nylon cartridge.
1. The piston is unable to move its primary seal past the tiny compensation port. There is no leeway. Before the brake lever is applied, the seal is directly behind that port. The instant it moves, the seal moves past the part cutting of any chance for the pressure to go anywhere but the brake caliper pistons. When the brake lever / pedal is released - that primary seal must be able to return fully. If there is any build up of rust/crud that prevents that free movmement, the pressure cannot be released.
2. Either of the two compensation ports can be clogged with gunge from crystalised brake fluid getting too wet. If fluid hasn't been renewed every two years at the very most, and the SMC piston has not been exercised when bleeding - then this build up is almost inevitable.
3. The bore itself may be distorted. It may have been knocked or the bike may have fallen against a kerb. A round piston is not going to be able to move easily on an oval bore.
4. The spring may have broken.
5. The bearings in the fork legs may have seized, preventing the smooth movement of the SMC Braking will overcome that, but the SMC will not return by itself. Someone may have used liquid loctite on the caliper bracket bolts, and it has flowed onto the bush that goes into the roller bearings.
6. Corrosion or scraping inside the bore - preventing the piston from moving easily.
7. Corrosion on the outside. Many people claim that they use silicone grease under the rubber boot. They never say how they put the boot back so that it doesn't let water in again. It is not possible to just shove it in. It will not seat properly. If that seal lets water in, it will get into the chamber between the spring clip and the cylinder itself. The secondary seal will stop it - but the bore will rust preventing the pison from returning fully.
You may well find that even with a thorough clean out that there is still soemthing amiss with an old SMC.
I reckon that one day, the SMC on an old bike will need replacing. I reckon that now might be a good time to get one - if they are still available. Having said that, the 1100 SMC seems less prone to the problems of the ST 1300 - I guess because it is mounted (and looks) like the rear master cylinder. It is the linkage that presents problems - but that is very easy to clean up and rarely needs more than a new split pin.
Note the direction of that cartridge. The Mercedes logo faces down. I'm not sure what the sprung seal is for. It forces returning fluid through the compensation port - but it has to go through the first port anyway. Its possibly to make filling and bleeding the system easier.
All master cylinders are a one way valve. Fluid from the brake side of the brake line cannot get past the seal. But whent he piston is returning, fluid from behind the seal can get past. Have you ever wondered why when you fill the rear brake system with fluid, it comes out through the outer piston bleed valve just by pressing the pedal ? It is flowing past that primary seal in the SMC - easy as anything. Very much like a bicycle pump pushes air into the tyre, but easily apulls bag up the tube allowing more air to get in front of that 'leather' washer.
There are a number of ways in which the SMC can fail. All of them are to do with the SMC not allowing fluid under pressure to return to the reservoir. The only way that it can get back to the reservoir is through the tiny compensation port. In the SMC, there are two tiny compensation ports - and the fluid has to get through both of them. One is in the cylinder wall. The other is in that nylon cartridge.
1. The piston is unable to move its primary seal past the tiny compensation port. There is no leeway. Before the brake lever is applied, the seal is directly behind that port. The instant it moves, the seal moves past the part cutting of any chance for the pressure to go anywhere but the brake caliper pistons. When the brake lever / pedal is released - that primary seal must be able to return fully. If there is any build up of rust/crud that prevents that free movmement, the pressure cannot be released.
2. Either of the two compensation ports can be clogged with gunge from crystalised brake fluid getting too wet. If fluid hasn't been renewed every two years at the very most, and the SMC piston has not been exercised when bleeding - then this build up is almost inevitable.
3. The bore itself may be distorted. It may have been knocked or the bike may have fallen against a kerb. A round piston is not going to be able to move easily on an oval bore.
4. The spring may have broken.
5. The bearings in the fork legs may have seized, preventing the smooth movement of the SMC Braking will overcome that, but the SMC will not return by itself. Someone may have used liquid loctite on the caliper bracket bolts, and it has flowed onto the bush that goes into the roller bearings.
6. Corrosion or scraping inside the bore - preventing the piston from moving easily.
7. Corrosion on the outside. Many people claim that they use silicone grease under the rubber boot. They never say how they put the boot back so that it doesn't let water in again. It is not possible to just shove it in. It will not seat properly. If that seal lets water in, it will get into the chamber between the spring clip and the cylinder itself. The secondary seal will stop it - but the bore will rust preventing the pison from returning fully.
You may well find that even with a thorough clean out that there is still soemthing amiss with an old SMC.
I reckon that one day, the SMC on an old bike will need replacing. I reckon that now might be a good time to get one - if they are still available. Having said that, the 1100 SMC seems less prone to the problems of the ST 1300 - I guess because it is mounted (and looks) like the rear master cylinder. It is the linkage that presents problems - but that is very easy to clean up and rarely needs more than a new split pin.
Note the direction of that cartridge. The Mercedes logo faces down. I'm not sure what the sprung seal is for. It forces returning fluid through the compensation port - but it has to go through the first port anyway. Its possibly to make filling and bleeding the system easier.
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