I haven't been able to find a cut away view of the PCV ....
It is out there somewhere. Saw one a long while back but I think it was on a GL site, don't remember seing one here. As you say, somebody is also bound to have taken one apart and taken pics as well.
So, the bleed down spool shift chamber port [for lack of a better word] whether it's along the spool or on the end, must be upstream of the point of closure in the PCV where it becomes exposed to an increase in pressure. It probably shifts back on a pressure drop via opposing spring force [guess]. I'm thinking somewhere along the spool in a separate chamber, when the spool does a high pressure shift the rear outer piston chamber becomes exposed to a piston or ball in another spring loaded chamber through a calibrated bleed port, the design of which determines the rate of effective hydraulic pressure drop at the outer brake calipers.
Your hamster has to take this pressure drop through calibrated bleed port concept out of your head. That's good for a shock absorber where there is actual fluid displacement from one chamber to another, but not for a brake system where pressure is the only thing propagating through the lines, with no volume displacement (of course there is some displacement, due to line expansion, fluid compressibility, air bubbles and very small piston motion and pad wear, but it is small and not exploitable via calibrated orifice).
Your hamster should let you know that it is simpler than what you think.
The pressure reduction on the rear occurs very simply as the spool moves back and expands the effective volume of the isolated rear segment of the line. Really as simple as that.
And just as you describe, the spool is pulled back by a piston activated by the SMC pressure on the upstream side of the shut off and working against a spring.
The higher the SMC pressure, the more it pushes against the spring, the more it pulls the spool back, the more the rear line volume expands, and the more the rear pressure decreases. That's really all there is to it.
And conversely, if you slowly let go off the brake, the rear pressure will build back up to plateau value as the SMC pressure goes down from its emergency braking level. So you will regain some rear braking power after you let go from a handful on the front.
Am wondering what might happen when somebody cooks the rear (I hear it happens!) and pressures from to the fluid thermal expansion generate forces against the spring side that may exceed design specs?
I'm going take one of my R410A High Side Gauges with a 800 PSI range, pull 29.92" vacuum, prime the bourdon tube solid with DOT4, connect it to the rear caliper bleed port and have my lovely assistant from the studio audience apply a gradual progressive pressure on the brake pedal.
800 psi might be just right. Chances are the shut off will occur just below this value and you might be able to observe the plateau, where you keep increasing the load on the pedal and your gauge pressure remains constant.
However, am not sure you can generate enough upstream pressure with only the brake pedal to go up to where your gauge pressure will start to go down. I suppose upstream should be in the 1000 to 1500 psi range to get you there. You may have to use a pump to achieve that.
Or, maybe....
-- Strap the front brake lever to the grip, to make the front brake grab the rotor.
-- Weigh down the back of the bike to get the front wheel off the floor.
-- Run a tie down strap between the front wheel and rear wheel and start ratcheting to rotate the front wheel
-- With the front caliper being locked on the rotor, this should engage the SMC and maybe help get to the pressure you need to reach the bleed down stage?
Or anything else that your hamster may suggest to get there.
.... pressure drop in the outer caliper chambers must be fixed and ideal for only a narrow range of conditions.
Fixed it is. None of the sophistication you might get from a computerized system. Honda has decided how much braking power they will let you have on the rear, regardless of your load distribution on the bike. What they are leaving up to you though, is how hard you want to stomp on the middle rear.
Hope you'll soon introduce us to your lovely assistant.
Some people also do a good job of turning their heads 90 degrees and keeping their eyes looking straight forward. I had a student from Thailand taking the BRC1 a few years back. He had been riding in Thailand for years but needed to take the class to get his NY motorcycle endorsement. He looked straight at the ground doing u-turns. I tried to get him to turn his head but he just smiled and looked at the ground each time. He could also do it at full handlebar lock in both directions. I said who am I to tell him how to do it when he does it better than I can.
