Some better quality kits use a cone shaped sheet metal piece between the spring and the cup, crimped to the end of the spring to spread out the load a little. I like "speed bleeders" where the bleeder screw has a spring loaded check ball incorporated into it, and sealing material on the threads of the bleeder. The screw is loosened slightly, and when the brake pedal is pushed fluid and air is forced past the check ball. On the return pedal stroke the check ball prevents air from being sucked back into the cylinder.
Most people pump the brake pedal too hard and fast during bleeding. One or two slow strokes of the pedal is plenty to pressurize the fluid, followed by holding the pedal down while the bleeder screw is opened. Pumping the pedal rapidly causes air to be "homogenized" into tiny bubbles that don't purge well. If the fluid at the bleeder looks foamy, your helper is getting too energetic on the brake pedal! I've been known to use a piece of broom stick propped between the seat and the brake pedal to hold it down when I'm doing a one man bleeding job.
Jerry
