Vent screw
A bleed screw, or vent screw, is a valve in the shape of a screw, manually operated by maintenance technicians, that drains air and old fluid from hydraulic systems.
The screw is tapered at the bottom to seal the vessel that contains the hydraulic fluid . Their threads are preferably hardened , and their feet are usually soft for a better seal. The head is provided with a hole that allows gases and liquids to escape. Usually it is a hexagon screw with a rotatable hose connection in the form of a nipple on the head. The vent channel is a right-angled channel in the vent screw made up of two holes. One is located on the side between the cone and thread, the other leads as a tube through the shaft of the screw to the nipple on the head. The vent screws are only tightened with the low torques described in the maintenance documents . Higher torques lead to damage to the vessel and thus to failure of the hydraulic system.
Bleed screws are used on vehicle brakes on motorcycles and cars and high-quality bicycles with hydraulic brakes . To change the brake fluid , which has to be changed due to its hygroscopic properties and abrasion, the nipple of the bleeder screw is connected to a bleeding device with a hose. The venturi effect is used to suck the old fluid and air out of the brake system via the venting device, which is operated with compressed air , while new fluid is refilled and flows afterwards. There are bleed screws on every brake caliper , brake cylinder and on other hydraulic components with line ends in the system such as the anti-lock braking system (ABS).
Hydraulically operated clutches on manual transmissions are vented on the slave cylinder . On some vehicles, the clutch and brake have the same reservoir for the shared hydraulic fluid.
In the case of bicycles, torques of 4 to 6 Nm are usual for tightening the venting screws , in cars mostly less than 10 Nm, mostly only 6 Nm.