Brake Problems !! Any ideas?

1998 5.2 SLT rear wheel ABS only.

Pedal is going straight to the floor with very slight pressure.
(keep in mind, this just started with no warning of any kind. No sqeeks, grinds, rattles......NOTHING!!! ) ,

Brake light and ABS light are both on. I checked all lines as well as removed drums from both rear tires to see if there was any leaks, and I see nothing. I tried bleeding the lines and no air come out of lines. I tried bleeding master cylinder but can not find a bleeder bolt on it. I went to the junk yard and bought a new master cylinder and installed it. I how ever, did not have any PVT to seal it 100%. So now I know that I lost vacuum until I get more sealer. Not very smart, I know!!

My questions are:
How can I check to see if the ABS module is working properly?
Is it possible the the vacuum is bad? (before I replaced Master Cyl.)
Is there something stupid that I forgot the check?
Is there a sensor that could cause this kind of a problem?

Does any one have any ideas as to what could be the problem?

Any information will be useful..... THANKS

UPDATE!

Today I took the master cylinder back off. After I took it back off, I preformed a "vice bleed" on it to remove any air from the M. cylinder. I then cleaned off the rubber gasket, removed the old paint and debris from the vacuum and M. cylinder, applied some RTV sealer to the master cylinder, re attached the master cylinder, then attached the break lines to the cylinder.

I have not yet bleed the brake lines because it started raining outside. But I did start the engine and tried to see if there was any resistance in the pedal. I didnt notice any difference :( !!!!!!!

*NOTE* I know that some people in here probably do not know how to do a "Vice bleed" on a new master cylinder. I learned how to do this today. I will explain how it can be done. Please remember that you will only need to preform this step if you have air trapped in the mstr. cyl., or if you are replacing it with a new or newer one.

Some models I believe have a bleeder screw attached right on the mas. cyl., but if not this is how:

1. Remove all break lines that are attached to the master cylinder. (make sure not to get any fluid on the plastic grill or other plastic parts. Keep the fluid away from all paint as well)

2. Remove the 2 bolts that mount the master cylinder to the power break booster.

3. Using a vice, securely fasten the master cylinder (be careful not to damage the mstr. cyl.)

4. Use bleeding tubes (from automotive shop. Or you can make your own with 2 10"-14" break lines*This is what I did*) and thread them to the master cylinder.

5. Bend the tubes over and insert them into the reservoir. Make sure the res. is 3/4 full and place the tubes below fluid level.

6. Using a blunt tipped rod ( I used a socket extension), slowly press the piston in ALL THE WAY until it stops. You should see bubbles come out of the tubes.

7. Continue pressing in piston until you see no more bubbles.

8. After the bubbles are gone, there will be no more air in the master cylinder. Then remove from vice, remove the lines or bleader tubes, and re-install the master cylinder to the power brake booster.

After you complete the bleeding of your master cylinder and it is not raining outside you must bleed the brake lines to remove the rest of the air out of the system. (remember to start from the tire that is farthest away from the master cylinder and work your way to the tire that is closest. That way you will avoid any air from getting back into your hydrolic system.) I hope that this will help someone out in case you run into the same problem and want to try to do this yourself and save some money.

If I have any info wrong and someone would like to correct me, please do so. I am not a mechanic, Im just a do-it-yourself kind of guy and Im just learning. Thanks

If you have a vacuum leak the engine won't run properly it will run very rough. The vacuum canister behind the master cylinder is what holds the vacuum. It has a diaphragm in it which holds the vacuum. No vacuum is supposed to reach the master cylinder. The silicon will change the spacing between the master cylinder and vacuum booster causing some of your problem. Are you getting fluid when you bleed all the brakes front and rear.
The light will come on if the pressure is different between the front and rear.
I think you have air in the system some where.
You may have purchased a bad master cylinder.
Sometimes on abs you need a scanner to remove all air.

rhandwor,
I am getting fluid as I bleed the brakes from front and rear.

when you say, "The silicon will change the spacing between the master cylinder and vacuum booster causing some of your problem.", are you saying that the distance from the Booster pedal rod to the piston in the mstr cyl. is too far away now causing it to not be fully compressed??

I hooked up a vacuum gauge to the booster check valve to see how much Vacuum I am getting and I am reading a constant 17-18 in. Hg and was not loosing and vauum over a five minute period. Any other ideas? Thanks for your help!!

You are correct about the push rod length. They are adjustable you could go back to the yard. Put a ruler flat against the vacuum booster then put a metal scale against the ruler and measure the length. Adjust the one on your car the same as the one you removed the cylinder from.
I would remove the silicon first as 1/8 inch will make a big difference.

I just did my brothers back brakes because the pressure just dropped and it happened to be the rubber on the wheel cylinder was half off ( we just had an estimate to see what had to be done because i can do the work im just not that great at diagnosing so we think they did it so he would have to rush to them to get the work done.) but after the cylinder was replaced it was pretty much perfect. it still has a little sink to the pedal before they start working but its what he knew as "normal" i was wondering what the sinking was from? fluid levels are fine. all hardware,cylinders,and pads are good in the back i was going to tackle the calipers,rotors and pads on the front. so if that doesnt work anyone have any suggestions? the master cylinder did leak for a while but now for some reason its fine no more leaks it holds the pressure. any ideas? thank you

If the master cylinder did bypass it should be changed. They will work for a few days and then you have to pump the pedal to get brakes.