The Oldsmobile brake master cylinder performs two functions for your vehicle. Not only does this device convert mechanical motion to hydraulic pressure, but it is also where the fluid reservoirs for the braking system are stored. The brake master cylinder builds the hydraulic pressure in the braking system when the brake pedal is depressed in the passenger compartment. This is accomplished with the physical movement of the piston in the bottom of the brake master cylinder that is mounted to your Oldsmobile. There is a seal on this piston which holds the brake fluid in place so pressure can be built up in the system and sent out to the brake calipers and wheel cylinders via the brake lines. It is the seal on this piston that tends to fail causing the Oldsmobile brake master cylinder to become faulty and be in need of replacement. When the seal on the piston permits hydraulic fluid to seep past it as it moves, the pressure being built up by the brake master cylinder cannot be maintained. This is why the brake pedal feels soft and can stay in motion until it hits the floor when the driver of the Oldsmobile attempts to stop the vehicle. When this seal is ruptured, the brake fluid cannot be fully pressurized and the pressure cannot be maintained. Unlike other braking devices, when the seal on the brake master cylinder is ruptured there is no external leak of fluid that can be seen. This is how a driver will be aware the master cylinder is bad when pressure cannot be built up like when they have a broken brake line, but no fluid in on the pavement below the Oldsmobile.