A while ago I posted a query concerning some off behaviour on a friends braking system. The symptoms were that in spite of getting rid of the air in his brakes and the brakes feeling absolutely fine, over a period of miles, his left SMC calliper would start to chatter and his rear pedal would go soft. A few pumps of the pedal and his rear brake would return to normal. I spent a long time with him over the phone to make sure that this wasn't an air issue and produced the article that I posted here 'Getting all of the air out of the Brake System'. He worked on the brakes with a friendly mechanic - always ending up with the same result. Brakes initially OK, and would get worse over a few miles.
In the meantime, he did some tests for me and we decided that in spite of the SMC service kit having been replaced in the last couple of years, that the full SMC bracket would be required. Last week, he fitted the new bracket and everything is working properly and better than they have ever done. Yesterday, I received a parcel in the post. His old SMC ! So naturally, I took it apart and took some photos - which I thought I would share. Click the thumbnails to see a larger 800x600 image.
First - various views of the SMC taken from an ST1300A4 bike. Note the little drain hole at the left end of the master cylinder in the first pic - modified in the last photo which is from my ST1300A9
....... ST1300A9 -->
Prior to removing the circlip to release the piston from the cylinder bore. The circlip is particularly difficult to access - even with the unit off the bike on the work bench. The lack of light and rubber boot and the plunger being in the way hinder things considerably. Note that in the last photo, I gave up and removed the bracket and the boot in order to get a better view. This should not be done unless a new piston service kit is to be fitted. The securing bolt is set to the correct length at the factory, and the correct length is critical.
In the next sequence, the piston has been removed. The photos show the amount of crud that has built up behind the boot. This unit was greased behind the boot and the drain hole was kept clear. Nevertheless, this is a mess.
Here I have turned the bracket over and removed the inlet port cover plate, which is retained with 2 small bolts, which have thread lock applied. Underneath is a rubber ring and a small white cylindrical cartridge - the SMC inlet port valve and the relief port
The Inlet port valve and relief port cartridge. both sides have a very fine mesh gauze. I haven't yet dismantled it. The last two photos show the inside view of the inlet port and the tiny relief port.
These photos are just for info for anyone who is interested and may find them useful.
The fault ? Well the metal 'cup' at the end of the piston which is closest to SMC plunger was a very tight fit inside the cylinder bore. There was no build up of crud just inside the bore - in spite of the amount on the outside, and the metal cup didn't feel rough. When the piston was pushed inside the bore, it met with resistance once the secondary seal was inside - the metal cup was tight against the sides. When it was pushed in further, it eased up and would return part way under spring pressure.
This sounds like the classic situation which often results in the SMC not returning and causing the back brake to lock, but this wasn't the case - on the contrary, the pedal lost pressure.
As it stands at the moment, I reckon that the cylinder has had a slight knock and has become slightly oval. Although the secondary seal was doing its job the fluid was able to move back past the primary seal, resulting in the long pedal and chattering SMC bracket. My diagnosis might be wrong but its a working theory.