As we all know, a pedal drop is a clear indication that the brake flush was performed by an incompetent wrencher (and yes....I can feel it!)
I think that you miss the point that I was making.
You are right that air in the system will result in pedal drop as different parts of the hydraulics are out under pressure and released. I can use this to identify which section of the rear hydraulics the final air bubble is located.
There are two other kinds of pedal drop even if there is no air in the system.
1. Is caused by the delay valve. When the rear pedal is first pressed it does not allow pressure to the front right centre piston. The pressure in the delay valve actually shuts off the flow of fluid to the front right centre piston. The front right brake pad is not moved to press against the rotating disc.
A spool in the delay valve is held in place by a strong spring and it seals off the outlet so that fluid pressure cannot pass immediately to the front right centre piston.
When the pedal is pressed harder, then the strong spring yields to the greater pressure and fluid may then flow - the front right centre piston can move the pad closer to the disc. That movement / change of resistance can be felt at the pedal.
2. When braking
hard only with the rear brake pedal - I'm talking about in the garage, because you probably wouldn't do that on the road - only the left and right centre pistons are operated on the front calipers.
If you then squeeze the front brake lever to increase the braking force, that can be achieved by pressing them harder against the disc than was possible with the rear pedal / front center pistons. That relieves some of the pressure on the front centre piston and this can be felt under your right foot.
Both of these effects are exaggerated if there is air in the system - but getting rid of all of the air cannot eliminate that sensation under the pedal.
I cannot get my head around the notion that stirring up the fluid in the SMC line can affect the front
outer pistons. But it could affect the front
centre piston, with the same end result. The banjo union for the front left centre piston and the inlet port for the SMC are linked directly by a short hose, and both are fed from the same line from the rear master cylinder.
I'll think about that some more as clearly the solution fixed the problem. But as already described when one piston is moved in the caliper, that affects the entire caliper which in turn has an effect on the other pistons.
If anyone is unsure of that, think about what would happen if, say, the top left outer piston was stuck in its bore. It would not move at all. Apply the front brakes. Only the lower piston is moved.
As pads wear the result of this is that the pad is at an angle. The caliper is not a tight fit on those slider pins, so the caliper is now at an angle. What happens to the centre piston ? The piston will be pushed in slightly as the caliper twists when the front brake is applied, resulting in a 'long pedal' .
When the centre piston is applied by the brake pedal, it applies a central force on the pads which will straighten things out again. But what happens to the lower outer piston in that case ? That gets pushed in. Soft brake lever.
Another footnote - I wouldn't have thought of this, but I saw it on a motorcycle race and heard it from an ex racer - I think it was Steve Parrish who was commentating:
Rider pulls a slight wheelie and when it comes down he gets some head shake. Front wheel oscillating, bars shaking. Not quite slapping the tank, but close. Rider recovers from that, and then comes off at the next corner.
Commentator says something to the effect of: schoolboy error - he should have pumped out his brake pistons after a headshake like that.
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