There is no vaccum lines to the booster. The wheel cylinders engage. With the engine off you can feel them quite well and with the engine running you can if you pump the brake pedal. the wheel cylinders and master cylinder were all changed apprx. 13 months ago and the brakes worked fine afterwards. I've tried regular bleeding, a vaccum bleeder (from the wheel cylinder bleed screws) and a pressure bleeder (at the resevoir) to make sure there is no air in the system and in all cases when the bleed screw is opened the flow pressure of the brake fluid coming out is normal.