I didn't realise that I had contributed to this post before - but it seems to be populated with a few of my photos and diagrams - not that I mind that one bit, they just took me by surprise.
@martinpl In a post of 7th April 2020 (shown as post #57 on my screen) - you have reproduced one of my braking circuit diagrams and circled the SMC inlet port.
Underneath, you have drawn that one way valve.
But you have drawn it the wrong way round !.
[edit]
@martinpl has checked this out and come to the same conclusion. Post #57 is now showing the correct orientation [/edit]
I think in case anyone else looks in and thinks that your diagram is correct, you might want to put it the correct way round. Use this if you like.
More info and pics of the dodgy SMC and of the inlet port one way valve cartridge (and the above full sized diagram) are here:
A faulty SMC, taken apart, sawn lengthways, with labelled photos and a commentary. This SMC had a brand new service kit fitted not long before it failed again. A while ago I posted a query concerning some weird behaviour on a friends braking system. The symptoms were that in spite of getting...
www.st-owners.com
Note the photo of the inlet port in the above link, taken immediately after taking off the black cover. The 'O' ring is still in place. Note that this is the side with the white gauze. The other side has a blue plastic clip-on cover. I verified this when I removed my own SMC a couple of years later - not touched since new. Mine was spotless !!
But really, if your SMC has got to the point where this valve is clogged, it really is time to replace the SMC - and learn to look after it better than the previous owner !
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Where does it relieve the pressure to, if pressed to hard....??
The proportional control valve sits in the braking system for the rear circuit between the SMC and the two outer pistons of the rear caliper.
The SMC operates those two outer pistons if the bike is in motion (as the SMC rocks forward whenever the front left brake is applied - which means when either the front lever or the rear pedal is operated). ie when braking, the SMC will operate the rear outer pistons. When the bike is stationary, the SMC cannot rock forward, but the fluid from the rear brake pedal will operate the outer pistons in the rear caliper. If you want to see how the circuit operates, then the diagram that
@martinpl reproduced above can be found in the link below - intended for bleeding, but it shows what fluid is moving, which pistons are operated and where the fluid comes from.
A detailed look at the process of getting the air out of the Honda ST1300 brake lines. A sequence of diagrams of the brake circuits, colour coded to show which parts of the system are being accessed when each bleed valve is opened and explanations of where the trouble spots are. These are just...
www.st-owners.com
Where does it relieve the pressure to, if pressed to hard....??
The proportional control valve is essentially a piston which moves inside a chamber. Normally, the piston remains closed under pressure of a strong spring. However, when braking force increases to the point where the rear wheel would be at risk of locking up, the spring gives way and gets pushed back into its chamber, relieving pressure from the rear outer pistons. It is actually a bit more complex than that, but that is the gist of it. Basically if you press too hard, the pressure is released before the brakes can lock.
This is completely different from the system which relieves the pressure from the pistons when the pedal/lever is released or due to expansion of the fluid due to heat. That is the tiny compensation port that exists in every master cylinder - the front brake MC, the rear brake MC, the clutch MC and yes, the Secondary MC.
For more information on this, see this link.
The operation of a hydraulic master cylinder - as used for the front brake lever, rear brake pedal, clutch lever and SMC. Coloured diagrams and explanations. 1 - 'Relaxed' Brake Master Cylinder (ie Brakes are not Applied) The image shows the front master cylinder and the reservoir...
www.st-owners.com
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