I have a 1998 chevy s10 it has a good brake pedal with the engine off but with it running the pedal go'es to the floor and you can hear air at brake booster

check all the rubber bushes ,and check your universal joint on your prop shaft bearing may be worn.

The booster may be bad, or the vacuum line to it (or elsewhere) has failed. Do a complete vacuum hose check, they are almost twenty years old, time for new ones. A vacuum gauge is a very handy tool to have, comes with cool tips on how to diagnose engines with it.

bad master cylinder.. normal for pedal to get hard while pumping pedal with engine off.

has to be air,try bleeding without the brake pedal,just open
the bleeders and let gravity run the fluid out,takes awhile but works.
(master cyl.cover off )

IF U HEAR AIR WHEN PUSHING BRAKE ,,ITS THE PUCK ,ITS BEHIND THE CALABER

yes may need to bleed lines at master cylnder

If you hear air, it is probably a failed diaphram in your booster. When you say you had good pedal, that was with engine off, right. Then, with engine on it goes to the floor. Does it still go to the floor with engine off? If no, it is the booster. Let me know if this isn't it or you have more questions.

1988 chevy r 3500 1 ton changed out driver front brake caliper
brakes are soft peddle goes to the floor tried to bleed but no brake fluid will come please help

Ok the BRAKE BOOSTER is ONLY there to give you pwer assisted, otherwise NO POWER BRAKES. ( not anything to do with the low brake pedal ) Hopefully the junkyard master cylinder was in better shape than your original. AS far a bleedinda system, heres what I do. First with a1998 vehicle, I would go to ALL 4 wheels , OPEN & CLOSE slightly each bleeder to insure none of them are FROZEN, and they do freeze shut ENSUE THE MASTER CYLINDER IS FULL and CLOSED Now with an assistant sitting in the drivers seat, ready to pump the brakes, and ABLE TO HEAR YOU. Go to the furthest wheel from the master which would be the R/R. DONT touch anything yet. Tell the asst too slowly pump the brake pedal till it gets hard, and then HOLD THE PEDAL DOWN ( and dont let up on it until the pedal drops. If the ast lets up before the bleeder is closed you **** in air.Repeat at each wheel til the pedal stays hard. Hope this helps

have one person in the blazer pumping the brake pedal and then hold to the floor, crack the bleeder then tighten, release pedal, repeat till all air is out one side at a time, if this does not work then your ABS unit is bad witch if u follow your lines from your master cylinder u will find a black box with a motor on it, or the master cylinder itself is bad, it also can cause u to have no pedal, replace the master cylinder first, most likely cheaper.