Brakes get soft on hot days

I have an 89 Toyota 4cyl, 4WD truck with the 22RE engine. 218,000 miles and the engine was rebuilt at 187,000.

I have replaced two master cylinders and continue to have an unusual problem. On hot summer days the brake power will fall off to the point that I have to run the pedal all the way to the floor to get any braking power. The hotter it is outside, coupled with extended driving times makes the problem worse. On a cold starts, even on a hot days, I have good braking power but as the engine warms the braking drops off. This takes about 20 minutes of stop and go driving. I have bled the brakes and as stated I have replaced the master cylinder but I am suspicious of the brake booster and/or a possible vacuum leak. Also, when the brakes are not working there is a "studdering" effect in the play of the pedal. As if it vibrates down the shaft which pushes the master cylinder. I can find no fluid leaks either. I would like some help in troubleshooting this problem and diagnosis.

Thanks Brett