Ford Repair: brake pressure lost with engine high idle, vacuum lines, gm vehicle

Question
i have a 86 f150 4x2 with the 351w small block paired with a c6 3speed transmission. recently i replaced the transmission vacuum modulator do to it not shifting into the final gear. after replacing the engine ran at high idle and i lose brake pressure. i already checked the check valve for vacuum on the booster and it was fine. checked several other vacuum lines andthey were fine. then found one the front top of the motor behind the air compressor for emissions control that was plugged with a rubber cap so i took the cap off and engine idle lowered but still losing brake pressure. any ideas cause i know my way around an engine and this is really bothering me and making me pull my hair out. will be driving a gm vehicle until i get this problem fixed so please hurry :(

Answer
Hello Robert, I did some homework on your question. One possibilty is the vaccum booster is bleeding off from a waek diaphram in the booster. Second is to check the main hose from intake and the nipple on the intake where it draws vacuum for a leak. The engine mainly produces vacuum below 1000 rpms. You can block off the booster with adapter and use a vacuum pump with gauge to see if it holds vacuum and or if it bleeds off. Thanks Tony