I have a 99 Blazer 4x4 with ABS.
Brake pedal just went to the floor and car stopped eventually ... a bit scarey.
I've checked all the brake lines and the inside of all wheels and there is no sign of fluid anywhere. All pistons seem to be where they need to be with backing plates nicely visible.

I pumped the brakes quite a few times and same result ... but I'm not pumping fluid on the ground.

As exected I've also checked the reservoir and its full. Didn't see any leaks so its full.
I'm guessing that the master cylinder has given up the ghost ... truck has 470,000km so things wear out.

Any opinions?

In all probability it is the Master Cylinder. If you remove the Master Cylinder from the Vacuum Booster, check to make sure the Vacuum Booster isn't full of Brake Fluid.

Good Luck, Post back with your results.

I had one that did that. It was never determined if it was the abs unit or a line with a tiny rust hole. The abs was bypassed and when it got pressure immediately blew a front line. After looking closer at the line it had been leaking onto a frame section that cought all the leaking fluid so none was on the ground.

To add to my post the master was changed with no change in the pedal, It is a common problem for them though.

The repair is delayed until the weekend.
But I'd just like to add a reassurance that it is highly unlikely that there is a leak.
The reservoir is not displaying any amount of lower fluid level.
No leaks are visible ... even an invisible leak would likely show up as a lower fluid level.

The fact that I experience some pressure and the car does stop indicates that the brake master cylinder plunger is moving and partially doing the job.
The fact that the pedal moves eventually all the way to the floor suggests that the seals in the master cylinder are failing.

With the age and travel distance the truck has gone its not surprizing that a component such as the master cylinder would have reached its lifespan. I'll swap it out and hope that all the bleed screws are functional ... I'm a little nervous with the 2 front calipers as the rear ones are pretty new. I forget the last time I actually loosed those bleed screws on the front ... I guess I'll find out soon. It wasnt too long ago that I did a complete flush of the brake fluid ... couple of years ago so maybe I'll be OK.

Just found a small leak on the side of my radiator ... must have come from leaving my car at the airport for 6 days and getting back with -25C weather, not bad enough to worry ... just have to make sure that the orange reservoir is full or it will turn to gunk in the motor. maybe some stop-leak will prolong the lifespan a little more ... ya I hear all the experts cringing but at 470,000 km I'm looking at wearing out the present set of tires and then calling it a day. Too many loose parts ...

Just realized i never finished up this thread. never got around to the stop leak ... things appeared to have sealed themselves up on their own. Don't know why, but you can't argue with devine results ... lol. As far as the brakes go. Swapped out the master cylinder and promptly blew the rear brakeline while testing. was kind of rotten so i replaced the whole brake line. Spent some time bleading the entire system and now brakes are good.

As an added note I also put in 2 cans of freon substiture or whatever the stuff is into the air conditioning. I do this every year and seems to get me through the really hot 2 months of the summer. This year was 1 can of sealer and 1 can of freon stuff.

Whats left to do ... theres a bit of body work to seal some rust penetration near the fron window at the top. I have finally gotten a hole in the original mufler and exhaust pipe, this and a bad CO2 sensor needs replacing as I have a clean air check before December. Got a deal on upper and lower ball joints ... this is a second replacement so there are no rivets to deal with. Also need to replace 1 tierod end as there is a broken boot and no grease. All this should make us good for another 500K Kms. LOL like I can afford the gas for that.

Appears that the electric mirrors have failed somewhere along the way ... might have a look at that, and the passenger auto door lock which only half works ... always something to fix. Blazer really owes me nothing. I just keep it going out of pride to see how far it will go.

If I really get inovative I'll have a crack at getting the 4wd to auto engage again. that depends on how rotten this second actuator really is.