It’s very low risk to not open a bleeder while compressing the piston if you keep up with maintenance. If you never flush your brake fluid in forever like a former GF I had then it can cost you. Doing a brake job on her old VW cost me almost a grand for a new ABS valve assembly after pushing mud like brake fluid back into the system.
That is the only reason I crack a bleeder every time. And regarding getting air in the system from cracking a bleeder, that is almost impossible to do unless the reservoir runs dry or somebody is releasing the brake pedal with a bleeder open. In the old days we would gravity bleed the brakes. Open all the bleeders, turn a bottle of brake fluid over in the reservoir, and come back overnight to do the final bleed. The bleeders are well below the reservoir and gravity just won’t allow the air into the caliper while there is fluid in the system.
Sent from my iPad using Tapatalk