brake problems

my 98 blazers brakes have been acting up. the pads and rotors are only a few thousand miles old. i noticed the pedal has been going down. its all the way down to the floor before the brakes engage. the fluid level is right up there and the pads are fine (no indicator squeal) the brake light is on because the pedal is so low...is it the master cylinder? how expensive and how hard to change? if not what should i check? i would say air in the lines...but the brake job was done awhile ago and how would air get in the lines if the system has been sealed up for so long? thanks in advance

Brake fluid level?
Red or yellow brake light
If no fluid leak check master cylinder.

Was the system opened up when brake job done?
Did the low pedal start before brake job or when brake job done or after?

fluid level is fine so i dont think theres any leaks. and the brake light is red

fluid level is fine so i dont think theres any leaks. and the brake light is red

If no fluid leak check master cylinder.

Was the system opened up when brake job done?
Did the low pedal start before brake job or when brake job done or after?
Does the pedal have a seap down on a light push down?
Engine off do you have a full brake pedal?
Rear drum or disk brakes?

"Was the system opened up when brake job done?"
yes because the calipers and rotors on the front were changed, but it was done about 9 months ago...and there wasnt a problem until now
"Did the low pedal start before brake job or when brake job done or after?"
after about 9 months after the last brake job
"Does the pedal have a seap down on a light push down?"
no it pretty much just drops to the floor without much effort...the brakes do engage and the truck will stop but the pedal is all the way to the floor...little scary but it is drivable at the moment
"Engine off do you have a full brake pedal?"
engine off when cold..pedal feels low. after its been running and then shut off pedal is high and feels fine
"Rear drum or disk brakes?"
rear disk brakes
thank for your help...i cant remember what i did before the internet...this forum has helped me out a lot through the past few years its been on here:)

I would go for a new master cylinder no rebuilt.
Bleed cylinder before installing.

The brake system on it requires the use of a brake bleeding capable scanner.
Like a GM scanner or snap on or other capable ABS system scanners.
Good luck and let us know how it goes.
MT