1990 Chevy 1/2 ton Master Cylinder Bubbling

[wsmm]

While trying to diagnose my brake light coming on I noticed something I haven't noticed before. The truck is a 4x4 with RWABS. When I first apply the brakes I may get a warning light on the dash and the pedal feels a little low, pressing on it again turns off the light and the pedal seems more firm. Had my wife press the pedal while I was under the hood looking for clue. I noticed the reservoir for the front brakes (the rear most reservoir) would bubble like air was entering from the bottom or that fluid was being forced back into the master cylinder. I also noticed that the front of the power booster was moving outward a bit. It did not matter if the engine was running or not. I also now a steady brake failure light and can not pull any brake error codes.
Any ideas of that might be wrong or how to fix it?
Thanks,
Bill

[texas sub]

if youre getting bubles that means that there is an air leak some where and the boster should not move at all. the bolts holding it probably came loose or broke. sorry to hear that its a pain getting to the top ones

[ronierogers]

currently suffering the same problem and i dunno what to do. i also think that this problem is not a do it yourself thing so this week i will be just looking for an independent mechanic that could fix my ride. it will be more advisable to bring it to your dealer because they know best but that is only my last option. please update me what did you do to fix it. i really dont have any idea where to start even looking for Chevrolet Parts is tough.