Question about master cylinders

[Constant Mesh]

https://www.tegger.com/hondafaq/mastercylinderreplace/static-schematic.gif

Looking at this diagram I'm wondering how additional fluid is transferred to the hydraulic system when you're bleeding the system, etc? This is a two circuit brake unit but a single circuit clutch system is almost identical but with one less circuit.

Assuming it's the FJR's clutch master cylinder and you're trying to bleed entrapped air from the system. You pull the clutch lever back and hold it and then open the bleed bolt releasing fluid and maybe some air bubbles. Then you close the bleed bolt.

As the lever is released and returns to its normal position how does additional fluid get added to the hydraulic circuit? You just released some fluid and air so the fluid volume is now less. As the master cylinder piston moves back to its normal position a vacuum would develop as the system volume increases.

Does the piston allow fluid to flow past it (left to right, high pressure to low pressure) as it moves back to its normal position?

Obviously once the piston reaches its resting position the fluid intake and return port is opened to the reservoir and additional fluid can flow from the reservoir to the hydraulic circuit.

But what happens before the piston has moved far enough left to uncover this port? Does fluid flow past the piston seal like a check valve?

 

[wfooshee]

When the piston is retracted, at rest, the fluid intake port allows fluid into the system. As braking is applied the seal passes that intake port and pressurizes the fluid in the lines. Any lost fluid is replaced by gravity when the piston retracts past that inlet port.

As far as what happens before, then yes, pressure is removed from the line, the pistons at the caliper will retract as allowed by the seals relaxing. If it actually makes a vacuum, well, that just helps the fluid intake port fill the system.

 

[RaYzerman19]

I can't say for sure....... perhaps the caliper piston pushes back some fluid, and/or when you release the brake lever there is enough pressure still existing in the system to push the master cylinder seal back past the open port, allowing new fluid in if required.....

 

[Constant Mesh]

I did a bit of research and discovered that fluid does indeed flow back past the piston seal to prevent a vacuum from occurring. The piston has a small number of grooves around its circumference which allows fluid to flow past it and the seal into the hydraulic circuit.

The term for this is recuperation. See Figure 2-3 below.

https://books.google.com/books?id=vV7Ols1n2TsC&pg=PA31&lpg=PA31&dq=master+cylinder+recuperation&source=bl&ots=SrLwfIO6NB&sig=TND8t1Mg12U-nKAGMh_gb2pQ7Ag&hl=en&sa=X&ei=IRjIUp6DNKTR2QXF3YHYDw&ved=0CEUQ6AEwAjgK#v=onepage&q=master%20cylinder%20recuperation&f=false

In the FJR Service Manual the drawing showing the internals of the clutch master cylinder shows the piston/seal having grooves around its circumference. I assume the grooves are for fluid flow around the piston.