has anyone replaced a brake booster i have a 99 blazer or suggestions of what else it could be. my brake pedal got real spongy and the brake light comes on, so i replaced the master cylinder and bleed them still same thing happening.

 

maybe the vacuum line to the booster, but its usually a good deal of effort to the pedal when the booster fails. spongy sounds like air, check your lines, maybe air is sneaking in.

 

Do you have disc or drum rear brakes?

As rooster says, if your booster has failed, you will have a firm pedal that requires a lot of effort to apply the brakes...not a spongy pedal. A low or spongy pedal is caused by air in the system or brake shoes out of adjustment.
I have also seen a spongy pedal caused by cheap, soft brake pads.

Did your problem start all of a sudden or did it happen over time? Have you checked all the lines, hoses, calipers and cylinders for leaks. Was the resevoir low on the master cylinder before you changed it?

 

its been for awhile about a year ago i put ceramic pads on and a little while after my problem started, and i see no visible leaks on the lines and reservoir was full

 

maybe a problem with the ABS.

 

Do you have drum or disc rear brakes?

 

all disc brakes

 

If your brakes are still spongy, I think you still have air in the system and need to do another bleed.
Buy a litre of brake fluid and do a full brake fluid flush.