Updated: Aug 11, 2021
The air from the foot control drives pneumatic handpieces (of course) but is also used to activate the coolant to the handpiece (both air and water).
Just as with a toggle used to turn on a secondary valve, the air pressure from the foot control activates a secondary valve to cause the water to run. This air needs to exhaust (relieve) so that your water will then shut off (just like with a 3-way toggle). This is why you can hear an audible "hiss" when you take your foot off of the foot control. A failure within the foot control valve (generally of an O-ring, but there are other parts that wear) can allow air to escape out of the exhaust hole constantly causing the foot control to "hiss". If you experience this, you will likely need to rebuild your foot control. Most manufacturers can provide rebuild kits with the precise components that may wear inside your foot control.
To rebuild the foot control, first shut the unit off and depress the foot control to exhaust all air from the lines. Next, turn the foot control over exposing the screws that hold it together. There will be four screws around the perimeter of the foot control holding the dome-shaped cover on. Remove only these screws. There will also be a pair of screws at the center of the foot control, do not remove those at this time. If your foot control has a "shroud" (a trapezoid-shaped area where the tubing enters) there will be screws holding it on as well. Again, this is not something you should need to be concerned with so you can leave those screws alone as well.
Once you have the necessary screws removed, you can then flip the foot control right-side up again and remove the disc-shaped top (or "dome" as it is sometimes referred to). This will expose the inner mechanism.
Shown above is a common configuration of A-dec foot control blocks. The A-dec foot controls are unusual in that they normally have two square blocks separated by a diaphragm. Most brands will only have a single block (the one on the left in the diagram above). Under typical circumstances, this is the block you'll want to "rebuild".
As you can see, the block consists of several basic components: 1 main block (with barbs to attach the tubing), 2 spools with O-rings, a large spring, a plastic washer and the main stem (which includes a snap ring). A manufacturer-provided rebuild kit will include replacements for all of these components except the block. Replace them all and lubricate with a silicone-based gel lubricant.
Often, simply disassembling the block, cleaning these components and lubricating with a silicone-based gel lubricant is all that is necessary to restore normal function. It's up to you if you want to try this before ordering a rebuild kit, or if you'd prefer to have the kit on hand before opening up your foot control. To a great extent, this depends on your own personal comfort level and how long it will take you to perform this task.