Odd Brake Behavior - Help Please

I have an odd brake problem that I’d like help diagnosing (without needlessly throwing parts at the problem). I have a ’94 4.0 5/spd. with ABS. The first few times I apply the brake (after she sits a few hours or overnight) the pedal feels like it’s applying itself and abruptly stops the jeeps (even with very light pedal pressure). It will let up if I take my foot off the pedal, however. Sometimes it applies them hard enough to chirp the front tires. The ABS system seems to work fine and displays no trouble codes. After I apply the brakes a couple of times or pump them a couple of times before starting off, the problem goes away until she sits again overnight. Any ideas? I’ve never seen a worn booster give those symptoms, has anyone else? I was thinking about pulling/plugging the booster vacuum line first thing in the morning to see if the problem occurs with without the influence of the booster. Has anyone experienced this problem before?

Also, the brake fluid and pads are only 1 year old.

Definitely try pulling the vacuum line to the booster. The ABS won't actually pull the pedal down.

Can you determine whether the entire brake system is doing this, or whether it is just locking a wheel? Rear wheel locking under conditions similar to yours is very common. In my experience it usually means the brakes aren't self adjusting properly and need to be adjusted or fixed.

I'll try pulling/plugging the booster tonight and pump the brakes a few times to bleed the reserve air out of it and let it sit overnight.

It's probably all the brakes that are near locking up (not just the front) when I first apply the brakes in the morning, but I definitely feel the pedal being pulled down abruptly until after a few brake applications. After I pump the brakes a couple of times, they work great until she sits again for at least a few hours.

Something may be shorting out the sensor (or just a bad sensor) and triggering the pump to activate the brakes. This is covered under the lifetime ABS warranty through Jeep, but only if you don't fool with it first! (well, the sensors aren't, but ABS problems otherwise are covered)

I would think that I'd feel the sensor pulsing if it were being activated. Wouldn't the sensor want to resist the pedal being applied too far, not actually help it apply the brakes? I think the ABS warranty only covers the first generation ABS (up to 91 or 92) but it's worth checking into. I want to eliminate the booster first as my first instinct is that it's starting to leak somewhere and just acting up.

Thanks for the ABS fuse idea, which turned out to be the easiest way to diagnose the problem. I've now narrowed it to the booster so I guess I'll be replacing it in the near future. Thanks again for all the help.