1991 Toyota 4Runner Weak brakes, 3 mechanics could not fix.

The brakes were great until I replaced the front pads. It feels to me as if htere is no vacumn assist. I bled them several times, no change. My wife to it to a repair shop where they put in a new master cylinder, a used vacumn booster, bled and adjusted the brakes still no change. I took it to a Toyota specialist (factory techs) who kept it for 6 weeks, but still could not find the problem. They did agree that the brakes don't feel right and supposedly checked pressures, calipers proportioning valve etc. They were so frustrated they gave the car back, no charge. This car does have rear anti-lock brakes. I have heard that some early versions would retain a memory so I tried disconecting power for several hours and put a new fuse in. All I can think of is that the booster is bad. The brakes aren't mushy they just wont lock up and I have to apply alot of pressure in a panic stop. Before all this they were some of the most sensitive brakes I have had. Please if anyone has any suggestions I would be so grateful. This is my 18 year old daughters car that she loves, but is afraid of now. Thank you for your consideration.