1986 Isuzu pickup. While holding the brakes, lose of pressure and noise from master cylinder,not loud noise

If the hydraulics were leaking, then you would loose fluid and there would be no noise.
Since you did not say you were losing fluid, and you said there was a noise, it is probably the power brake booster instead. You could plug the vacuum line to the power brakes to see if the sound goes away (but brakes will be hard to push)?
The power brake booster can be rebuilt with a new diaphragm, but a new one should be about $75.
They are usually not hard to do because you don't have to remove the brake lines from the master cylinder.
Just unbolt it and bend the lines a little in order to get it out of the way.