Any RWAL ABS brake gurus out there? (Long)

Hi Guys,
I have a '90 S-10 Blazer 2WD 4.3 that is about to drive me crazy! The brake light keeps coming on and I cannot figure out for the life of me what the problem is.

As I am describing this keep in mind that I have already replaced the master cylinder,ECM,combo valve assy and the ABS valve assy. all to no avail.The master cylinder is a brand new unit and the rest of the parts were used from another Blazer that I found in a wrecking yard that appeared to be in decent shape and the problem has been the same before and after all of the above mentioned parts were installed.

I have placed a jumper beetween the A and H terminal on the ALDL and the code I get is 3 fast flashes followed by 1 long flash ( 4?) but since I have already replaced all of the sensors that send signals to the ECM from the brake system as well as the ECM itself leaves me baffled even further.

The funny thing is that it does not come on randomly it only comes on during one of 2 scenarios:

A)When I start up the car first thing in the morning, I can pump the brake pedal as hard and as quickly as I want and it will not come on UNTIL I place it in gear and start to drive. Then the first time I hit the brakes after the car is set in motion it comes on. The thing that makes this particular scenario even stranger is that I can drive the car to work ( The system is losing NO fluid whatsoever),let it cool down over the course of my 8 hour shift,reset the ECM before I drive away AND IT STAYS OFF ! I can drive the car for the rest of the day in bumper to bumper traffic and it will not come on again until the next morning when I press the pedal for the first time with the car in motion.The only exeption to this is:

B) If I hold the pedal down with the vehicle in drive for extended periods of time ( like at a fast food drive thu) it will eventually come on.

The entire system was bled by myself and was done again by a mechanic that assured me there was no air in the system. The only possible strange occurance that occured during the bleeding process was that the rear brakes bled kind of funny.The fluid did not flow out of the cylinders like I am used to seeing with older brake systems.It came out in what appeared to be metered "spurts".Is this normal for an ABS system or does it sound like somthing is amiss?

Also, somtimes the pedal feels like it is dropping about an inch or so of travel before the brakes feel like they are really engaging. They don't feel spongy mind you, it almost feels like it would if there was a small hole in the power booster diaphram and me depressing the pedal was forcing the air out of the hole but the booster seems to be in good working order.

I apologize for the long post but i am about at my wits end with this. If there are any questions or data that I have overlooked please feel free to ask.

Thanks for the help,
Dave

first of all , when you say brake light , i hope you mean thebrake light ad not the abs light....could be something as the stupid brake fluid sensor somewhere on the master cylider , makin sense why it would only happen in the morning - cold brake fluid. if you havent replaced that stupid sensor , try , GM have always had probs with thos stupid sensors on the master and coolant bottle - when it does it , als try tapping on the sensor , sometimes itll jar it enuff to knock it lose and operate proberly. if it doesnt have one at all , well , sorry , lol , its kind of hard to diagnose over the net

If it is the brake light and you've changed everything else I would also look to the brake light switch at the top of the ebrake as well as both switches on the brake pedal itself...Start with the ebrake one first though..

Thanks for the replys,
In the '90 model the brake light and the ABS light appear to be one and the same. I did not mention it before but another thing that I did was to disconnect the fluid switch that is located on the combination valve to see if it was that switch or the ABS system that was causing the problem and the light came back on the following morning.