help brakes wont work

i have a 1998 lumina with a 3.1. the brakes will not pump up. there is no leaks and it is all the whell cylinders work fine. the brakes seem to work when they are on the stand but not enough to grab the car when driving.the peddal goes almost to the floor.could it be the master cylinder itself or the abs or brake booster

Have any repairs been done to the brakes? How and when did this start?

no repairs done yet cuz i can not fig out the problem. brother was driving it to the store and the brakes just went out. there seems to be back brakes but no front brakes. like i said no leaks and they seem to bleed alright but will not pump up to enough brake pressure. no cylinderes are leeking and none are sticking.

Sounds like M/C...boosterwould give you a hard, tall pedal with no assist if it went out...ABS is not active or part of the hydraulic system of the service brakes if not activated.

The piston seal(s) failed in the M/C, it's now bypassing pressure around them when you step on the brake..