I typed this into Google:

 

the difference between an open and closed hydraulic system

 

The primary difference is flow path and pressure control: an open system has continuous fluid flow through a directional valve and returns to the tank when idle, while a closed system only circulates fluid when needed, with fluid returning directly to the pump. Open systems are generally simpler and less expensive but less energy-efficient, whereas closed systems use variable displacement pumps for better energy efficiency and precise control but are more complex and costly.

Here's a breakdown of the key distinctions:

Open Center Systems

Pump Operation: The hydraulic pump runs continuously, pumping fluid even when the system isn't actively operating an actuator.

Valve Design: The directional control valve has an "open center" or "pass-through" design, allowing fluid to flow continuously through the valve when idle.

Pressure: Pressure is intermittent, developing only when a specific actuator is engaged.

Cost: Generally less expensive, often using fixed displacement pumps.

Applications: Found in systems like farm tractors, where constant fluid flow is acceptable and cost is a major consideration.

Closed Center Systems

Pump Operation: A variable displacement pump, used with a closed center valve, stops or reduces fluid flow when no actuator is engaged.

Valve Design: The directional control valve "centers" when no actuator is selected, stopping the flow.

Pressure: The system maintains continuous pressure even when idle, which is then relieved by a pressure relief valve.

Cost: More complex and expensive, requiring a variable displacement pump and more intricate controls.

Applications: Used in systems requiring precise control and high power, as they are more energy-efficient and generate less heat when idle.

 

are hydraulic bike brakes a closed or sealed system

 

Hydraulic bike brakes are a closed system, meaning the brake fluid is contained within the system of hoses and reservoirs, preventing contamination from water, dirt, or debris. This sealed design enhances reliability and power by eliminating cable stretch and contamination common in mechanical brakes, although it requires specialized maintenance like bleeding to remove air or fluid leaks.

How the Closed System Works

Fluid and Hoses: The system uses a special hydraulic fluid to transfer force through sealed hoses and lines.

Brake Lever: Squeezing the brake lever activates a plunger in the master cylinder.

Fluid Movement: The plunger pushes the hydraulic fluid through the sealed hoses.

Caliper Action: The fluid then enters the caliper, which clamps the brake pads onto the rotor attached to the wheel.

Braking Force: This action stops the bike, providing powerful and smooth stopping power.

Key Aspects of the Closed System

Contamination Protection: The sealed nature of the system protects it from water, dirt, and debris that can affect mechanical brakes.

Maintenance-Free (Generally): Once installed correctly, these systems are often low-maintenance because they don't suffer from cable stretch or contamination issues.

Potential for Leaks: Despite being sealed, leaks can occur through worn O-rings or punctures in the lines, compromising the system's effectiveness.

Bleeding Required: If air enters the system or a leak occurs, the brakes must be bled to remove the air and restore proper function.

can you have an open piston system within a closed hydraulic system

 

No, you cannot have an "open piston system" within a closed hydraulic system, because the terms describe mutually exclusive characteristics of the system's overall design and function. An open-center system continuously directs hydraulic fluid to the reservoir when not in use, while a closed-center system maintains pressure by blocking the fluid return and requires a variable-displacement pump or bypass valve to manage excess flow. A piston itself is a component, and its operational characteristics depend on the system it is part of.

Open System:

Flow to tank: In an open-center system, when no work is being done, the fluid flows from the pump directly back to the tank through the center of the directional control valve.

Pump type: Fixed displacement pumps are often used, providing a constant flow rate that is dumped when not needed.

Operation: Components are connected in series.

Closed System:

Continuous pressure: A closed-center system is designed to maintain pressure even when no work is being done.

Variable-displacement pumps: These systems often use variable displacement pumps with adjustable swashplates that can be moved to a zero-stroke position, stopping oil flow without needing a bypass valve.

Load sensing: They are also known as load-sensing systems, where the pump provides flow and pressure only when needed.

Why the distinction matters:

Component choice: You can't just use a closed-center valve on an open-center system; this will cause the relief valve to open, leading to overheating, according to Red Power Magazine.

System design: A system is generally designed from the start as either open-center or closed-center.

From what I can understand from the specification they are using the term "Open Circuit" to indicate there is a hydraulic oil reservoir and "Closed Circuit" to indicate no reservoir (on cable actuated, hydraulic brake pistons).

From what I can understand from the specification they are using the term "Open Circuit" to indicate there is a hydraulic oil reservoir and "Closed Circuit" to indicate no reservoir (on cable actuated, hydraulic brake pistons).[ATTACH type=full" alt="Screenshot_20250921-091758.png]64429[/ATTACH]

That BH-RS911 picture shows a spring behind the piston. That's how I suspect the middle one to be too, which would cause the issue I had.

 

In a brake with a separate reservoir, when the spring pushes the piston back, more oil can be sucked in from the reservoir, but when the brake is closed and doesn't have a reservoir, the oil gets sucked back from the pistons behind the pads (slaves), opening up the gap between them. That's the problem I was describing above. The only way to adjust the gap is either by adding more oil or adjusting the start point of the master piston.

I don't get what you're saying. These brakes are completely closed. Please can you explain?

I guess from the technical spew that has now covered the page you get the gist of the difference between the two.

So it isnt 'closed' as in all brakes are closed, or the fluid would leak out.

 

But main point is how the pistons work, so I've looked out a diagram where you can see the post inside the caliper body -fig2 and the parts(fig 9,10,11) that sit inside the piston in order to attach it to the caliper body.

 

 

On a closed system, there is no diaphragm, and the fluid is a held between the pistons at one end, and what is effectively a piston in the master cylinder receiver(Not the piston that is actuated by the lever)..

The top cap on the master cylinder has a thumb wheel on it, and by turning it one way or the other, you can advance the pistons/pads(as the pad wears) or retract the pistons/pads, if for example the system becomes hot and the fluid expands**pushing the pads into the rotor until it binds.

I found it best to leave a few 'off' turns so you could retract should you need to.

 

**On the closed system, (and remember this was 1996, and Hope was the only ones doing hydraulic at that time in the UK)

Heat was the big failing on the first brakes. The closed system meant when the fluid heated up and expanded, the only thing in the system that moves is the pistons, and the heat would expand it so much that the pads would lock onto the rotor, and the only way to release it was to open the caliper bleed valve and bleed out fluid till the pads spread apart.

 

Long downhills that were becoming common meant the closed system could no longer be used.

Enter the open system, that had a rubber diaphragm that allowed for this heat expansion

 

The actual operation of the pistons in and out is a result of the piston seal, not the movement of the fluid.

Edited September 28, 2025Sep 28 by AndyBike

I guess from the technical spew that has now covered the page you get the gist of the difference between the two.

So it isnt 'closed' as in all brakes are closed, or the fluid would leak out.

 

But main point is how the pistons work, so I've looked out a diagram where you can see the post inside the caliper body -fig2 and the parts(fig 9,10,11) that sit inside the piston in order to attach it to the caliper body.

[ATTACH type=full" alt="Screenshot (176).png]64436[/ATTACH]

 

 

On a closed system, there is no diaphragm, and the fluid is a held between the pistons at one end, and what is effectively a piston in the master cylinder receiver(Not the piston that is actuated by the lever)..

The top cap on the master cylinder has a thumb wheel on it, and by turning it one way or the other, you can advance the pistons/pads(as the pad wears) or retract the pistons/pads, if for example the system becomes hot and the fluid expands**pushing the pads into the rotor until it binds.

I found it best to leave a few 'off' turns so you could retract should you need to.

 

**On the closed system, (and remember this was 1996, and Hope was the only ones doing hydraulic at that time in the UK)

Heat was the big failing on the first brakes. The closed system meant when the fluid heated up and expanded, the only thing in the system that moves is the pistons, and the heat would expand it so much that the pads would lock onto the rotor, and the only way to release it was to open the caliper bleed valve and bleed out fluid till the pads spread apart.

 

Long downhills that were becoming common meant the closed system could no longer be used.

Enter the open system, that had a rubber diaphragm that allowed for this heat expansion

 

The actual operation of the pistons in and out is a result of the piston seal, not the movement of the fluid.

I'm not too sure what you're saying is right. All normal hydraulic brakes work the same. There's some sort of valve in the mster piston that lets air or excess oil back into the reservoir. Logically, it's only open when the brake lever is in its resting position, otherwise it wouldn't be able to force oil down the hose to the slave pistons. In that respect, the system is open to the air in the reservoir and oil can move out of the pressurised hoses into the reservoir.

 

If the reservoir doesn't have a bellows and is full to the brim, the system becomes closed because the expanding oil can't expand into the reservoir, so instead it expands the slave pistons and locks the brakes on. I had that happen once, so I had to take the reservoir cover off and let some oil drain out. There were no bellows in it. This is the reason that you always have bellows in the reservoir on cars and motorbikes. Some of the more expensive bicycle brakes have bellows too.

 

None of that is really relevant to these closed cable hydraulic brakes because they don't have a reservoir nor any hoses. The oil has nowhere to expand, and there's no way to compensate for pad wear, so the master piston lever keeps moving forward as the pads wear until it bottoms out and the break won't work. That's why you have to either put more oil in or change the pads.. There is a small adjustment on the slave piston pivot point, but it can't be enough to compensate for significant pad wear.

I'm not too sure what you're saying is right. All normal hydraulic brakes work the same. There's some sort of valve in the mster piston that lets air or excess oil back into the reservoir. Logically, it's only open when the brake lever is in its resting position, otherwise it wouldn't be able to force oil down the hose to the slave pistons. In that respect, the system is open to the air in the reservoir and oil can move out of the pressurised hoses into the reservoir.

 

If the reservoir doesn't have a bellows and is full to the brim, the system becomes closed because the expanding oil can't expand into the reservoir, so instead it expands the slave pistons and locks the brakes on. I had that happen once, so I had to take the reservoir cover off and let some oil drain out. There were no bellows in it. This is the reason that you always have bellows in the reservoir on cars and motorbikes. Some of the more expensive bicycle brakes have bellows too.

 

None of that is really relevant to these closed cable hydraulic brakes because they don't have a reservoir nor any hoses. The oil has nowhere to expand, and there's no way to compensate for pad wear, so the master piston lever keeps moving forward as the pads wear until it bottoms out and the break won't work. That's why you have to either put more oil in or change the pads.. There is a small adjustment on the slave piston pivot point, but it can't be enough to compensate for significant pad wear.

Agreed totally academic just on how brakes work and the different type there was originally.