The amount of pressure depends on how much FORCE you put on the piston.

If the piston in the above system has an area of 2 square inches and the FORCE is 400lb then the pressure is calculated using the formula as follows :

 400lb / 2 square inches = 200 psi

A smaller piston gives a higher pressure. If you replaced the piston with one with only a 1 sq inch area in the same system with 400lb of FORCE then the pressure exerted is 400 psi. Pistons can be used to multiply the FORCE in a hydraulic system. By choosing pistons with different surface areas any relationship with FORCE is possible.

A simple hydraulic braking system on a motorcycle is shown below. The pistons in this system have two different surface areas with the master cylinder piston having a surface area of 2 sq inches and the piston in the caliper has a surface area of 4 sq inches. With the same 400lb of FORCE being applied to the master cylinder the piston produces 200 psi. Remembering the hydraulic laws we discussed earlier : 

• Fluid cannot be compressed to a lesser volume, no matter how high the pressure

• Pressure is equal over all surfaces of the containing system

When looking at the two hydraulic laws above we know that the 200 psi in our example system will act equally on all surfaces within the system. The shell of the master cylinder, the shell of the caliper and the hoses that connect them both will have 200 psi acting upon them but they cannot move. However the piston at the caliper is able to move and will have the same 200 psi acting upon it.  

Interesting Fact

This is why it is important to replace the rubber hoses in your braking system with stainless steel braided. The pressure within the system acts on all surface areas of the braking system including the brake hose. This pressure when pushing outwards on a rubber hose can cause the hose to swell under heavy braking. The pressure is constant so you will still have the same pressure acting on the caliper piston BUT the amount of movement the caliper will have is reduced due to the amount of movement within the hose and also the FORCE acting upon the caliper will be reduced due to some force being used up swelling the brake hose.

As this piston has a 4 square inch surface area the FORCE it produces will be 800 lb. The area has doubled so the FORCE is doubled. 400lb of FORCE is pushed down onto 2 inches squared which then acts upon the 4 inch square surface. We must remember that only the FORCE changes in this system – the pressure remains the same at 200 psi. This is 200 per square inch so with the other piston having 4 square inches the FORCE produced has twice the surface area to work upon and so twice the FORCE in lbs.

To increase the FORCE on the caliper piston we can decrease the area of the master cylinder piston area or increase the surface area on the caliper piston. The reverse is true to decrease FORCE on the caliper piston. We can either increase the area of the master cylinder piston area or decrease the surface area on the caliper piston.

So that covers the FORCE at work in your braking system and we know that by decreasing or increasing parts of the system you can alter the FORCE produced at the caliper but that does not increase pressure. Pressure is the constant in the braking system and cannot be altered internally. If you can produce 200 psi at the master cylinder hydraulic law requires that 200 psi is produced at the caliper.

Interesting Fact

So the fitting of the smaller dash 2 hoses sold by some manufacturers makes NO difference to the pressure produced at the caliper as is commonly believed. There are no substantial benefits associated with the use of dash 2 hoses except that the manufacturer can charge you more for them. The only way to increase pressure coming out of the system is to increase the pressure going into the system. 500 psi in equals 500 psi out –  

• Fluid cannot be compressed to a lesser volume, no matter how high the pressure

• Pressure is equal over all surfaces of the containing system