The foot control or, more properly, rheostat is a key component to your delivery system. Air passes through the foot control when you step on it to drive your handpieces, but this is not all a foot control does.
All foot controls will have one main large (1/4” o.d.) line that feeds air into the foot control. This line will be “ribbed” and appears to be a seam running along the length of the line. This raised portion is there to identify the “air-in” line.
On most units (Marus, Beaverstate, DCI, etc.) the air outlet line from the foot control connects to a single large line at the end of the umbilical, which then connects to a routing manifold (#49-83). The routing manifold then splits the drive air to send air to the air coolant adjustment knob, an air signal to the water on/off toggle (and then to the water relay), as well as a line to the drive air port of the handpiece block. So, while stepping on the foot control running your handpiece (and allows you to control the speed of the handpiece) it also provides the air and water coolant to the handpiece.
On most A-dec units, the foot control has two blocks connected together underneath the foot control dome. The main block is simply drive air-in and out, the secondary block (often called the signal relay) also receives the drive air from the main block and splits it to send a signal through one small line to activate the coolant water, and occasionally a second small line for coolant air. Normally, none of these foot control lines are fed through the umbilical. They attach to color-coded lines in the umbilical, which then attach to a manifold in the unit for ultimate connection to the handpiece blocks. Many A-dec units have the manifold mounted directly to the assembly of blocks, depending on the specific model of A-dec unit.
If the foot control has a water on/off toggle (pictured in the diagram below), the unit must be configured to accept it. The additional split of drive air for water activation is not present at the routing valve in the unit (the outlet port is blocked). Instead, there is an extra outlet port on the valve in the foot control, which feeds through the toggle and then normally attaches to an extra line in the umbilical (often green or green with a black stripe), then feeds directly to the water relay (this is an air activated valve inside the unit that turns the water to the handpieces off and on). This way, the water is not automatically turned on every time the foot control is depressed. Additionally, there is no water on/off toggle on the unit.
For foot controls with a button (pictured in the diagram below), there is once again an extra outlet from the main foot control valve that feeds to the button when the foot control is depressed. This simply changes where the drive air is split. Sometimes the button is used for coolant air that is fed directly to the block (bypassing the needle valve for a great burst of air through the coolant ports). It can also still be fed through the coolant needle valve to allow for adjustment of flow. A-dec normally uses a green line with a black stripe in the umbilical for coolant air (same as water activation line); other manufacturers have a tendency to use different colors. Of course, the button can also be used to send a signal to an air activated electric switch for an ultrasonic scaler or electro-surge, etc.
Ultimately, the only difference in all the foot controls is where and how the drive air splits and what sort of additional on/off control there may be for the split air. For foot controls with accessories and multi-line A-dec units, the split occurs within the foot control and all the lines are ultimately hooked up the same (extra small line of 3 hole foot control is always for water coolant activation, unless the foot control has a button). For two-line non-A-dec units, the split is in the unit.
Last of all, there are occasional exceptions to the rules. For example: Beaverstate's two handpiece units tend to have the foot control tubing run directly up to the unit and attach to the routing valve directly.
As you can see, the air from the foot does much more than just drive your handpiece. When replacing your foot control make sure that your new foot control is properly configured for your delivery system, otherwise you will lose key functions.