Toyota Repair: 91 4Runner v6 vacuum and performance, head gasket leak, plenum chamber

Question
I had a head gasket leak and replaced both. The engine now runs poorly, i advanced the timing and it ran ok. I verified everything till blue in the face. After the 1-3 min warm-up; shifting into gear, especially fully pressing the break will kill the engine. Depressing the break takes vacuum from 18" down to 13". I remembered setting the timing belt! One cam was perfect but the second seemed 1/2 notch off. After 4 attempts I called it good. Can you explain how depressing the break would reduce the engines vacuum. I'm pretty sure, but would like to fully understand.
Thank you for your help.

Answer
The brake vacuum boster receives constact manifold vacuum from the intake manifold, when the brake pedal is depressed the vacuum acts on the brake booster diaphragm to proveide brake power assist, if the idle drops or the engine dies only when the brake is depressed then there is a leak in the brake booster, if the idle is rough and there is a loss of power without using the brake then there is a vacuum leak on the engine side which will be aggravated even more when using the brake so I doubt if the brake booster is the problem, it's more likely that there is a vacuum leak due to a hose that was not connected or connected in the wrong place, check the PCV hose under the plenum chamber on the right valve cover, make sure it's not cracked or disconnected.