Pumping the SMC can be carried out when the PCV bleed valve is open, or when the rear bleed valve on the rear caliper is open.
i believe that it is important to do this to:
- exercise the piston through its full range of travel
- to sluice out the old fluid which lies between the inlet port and the secondary seal
- to help to purge air that is trapped behind the piston, and in the mouldings on the piston body
- to feel for any binding or restrictions that may be developing
Bleeding involves removing the SMC and tilting it so that the outlet port is uphill from the inlet port - to encourage bubbles to float up to the outlet port. 0 to 15 degrees tilt.
With one of those bleed valves open, and the bracket unbolted from the fork leg, the SMC can be pumped by pushing and pulling on the plunger.
If the SMC is still bolted to the fork leg, the bracket can be moved by hand. I curl my fingers around the fork leg and squeeze the bracket with the heel of my hand. But simply pushing the bracket is easy enough if one if those bleed valves is open.
With the system properly bled and all bleed valves closed, pumping the SMC is harder. The amount if movement is minimal. A tad more than 1mm. Anything more and either there us air in the system or there are problems developing with the SMC. Or you forgot to reinstall your rear brake pads ! That small movement is enough to lock the rear wheel in the workshop. It should release the rear wheel instantly you release the pressure on the SMC.
Pumping the rear pedal will pump fluid through the SMC. Pumping the SMC helps to make sure all of the old fluid is expelled. I use a vacuum pump on the PCV bleed valve when I pump the SMC, but alternating SMC pumps with brake pedal pumps a couple of times will do the job.
One other trick for testing the release of pressure through the compensation port in the SMC:
With the the rear pistons clean and shiny, and a tube attached to the front left lower bleed valve, push in the rear outer pistons. This should expel fluid through the bleed tube through the compensation port in the SMC. It is easier than pushing fluid up to the rear reservoir, as that involves a much longer length of plumbing, and two tiny compensation ports to negotiate.