I found a bunch of good info some time back, and collected it into one thought process.
http://www.4g61t.org/forum/posting.php? ... 2&p=300492
British 92-95 Mirage was listing 1.3/1.6/2.0D engines with 4.5 or 6:1 (ABS) booster ratios, same for the PDF FSM listings for 93-96 Mirage (I assume USDM/CDN).
I think the larger the total surface area of your caliper pistons, considering the small size up in master cylinder bore, the more booster pressure (and heavier weight the cars equipped with them were) was equipped on the cars. Watch out for ABS boosters that were listed as a dual circuit style, because that gives different booster ratio front and back! Just to add to the confusion. It's tuning your brakes, as if tuning the engine wasn't finicky enough.
Maybe the fulcrum point or lever ratio is different inside the 4G chassis and this equation is thrown slightly out of whack.
Engine vacuum will directly affect your brake booster operation and how much "draw" it is able to work with as you enter decel and stomp on the pedal. With a car running low vacuum (cams/tuning or other maladie), sitting at the traffic lights or low speed operation can be drastically affected. Race track with high RPMS and throttle snapping shut, can yield high vacuum readings, but city driving likely won't if the car isn't setup for that environment. It has been a long time since I have been thinking about variable like this. Main reason I tried to stay with mild cams, and keep my vacuum leaks off the chart. Now my master leaking out fluid past the rear seal, has me worried about the booster. I'll check it visually when the master is off, but a booster function check is pretty easy to perform.
Put foot on pedal and start engine. Pedal should drop a little bit and stay at that level with continued effort at the same output.
Foot still on pedal, shut off engine and pump pedal until it gets really stiff and no longer provides added boost effect.
If the pedal dropped + stayed at point one = good.
If the pedal got really stiff to move after 1-3 pumps, then = good.
This proves the booster can provide boost/hold+allow use of vaccum. Check valve should be good if vacuum is held in the booster after engine is shut off. Remove hose and it should allow a "whoosh" noise as air enters back in quickly!
LILEVO, what is your master cylinder diameter? 2G Eclipse ABS or NON ABS? What year range?