The other day I was driving the truck ('99 4dr 4WD) w/o incident, parked it to run into the store and when I came back and started it, the parking brake light was lit up on the dash (P Brake was not engaged) and my pedal went to the floor. No sound of air, and there doesn't appear to be any loss of fluid, but brake performance is very poor...wondering if this could be a bad booster or master cylinder? As an aside I pulled the ABS fuse a few months ago as it was displaying classic symptoms of a bad ABS Module but until now did not change regular braking behaviour-any insight would be appreciated before I start throwing parts at it...
Thanks

 

What were the classic symptoms of a bad ABS Module that you had?
It is possible that those symptoms may be related to what is happening now. Poor pressure from a failing master cylinder or power booster can cause ABS performance issues for example.

The brake light comes on whenever a problem is detected in the brake system that is not ABS related as that will show the ABS light instead.

A suddenly soft pedal with a brake light usually means either a fluid leak somewhere or a bad master cylinder.

The one m/c test is to pump the pedal a few times then keep pressure on it.
Then start the car. The pedal should drop a bit but then stop. If it continues to slowly drop as you maintain pressure on the pedal, then you are likely in need of a m/c replacement.

 

Thanks for the reply-with the iginition off and pump the brake it firms right up-as soon as I start the truck with my foot on the pedal it sinks right to the floor and no loss of fluid-time for a new m/cylinder I guess...
As for the earlier symptoms of the ABS module, when I would be braking, just as I was coming to a stop, the ABS would kick on, always, even on dry pavement. I thought I read somewhere that this could indicate a bad module...I pulled the fuse and no longer have the issue (Or ABS unfortunately)

 

If you have no external leaks, it does sound like your master cylinder is at fault here. You may want to try a good bleed process on it though before condemning it.

As far as the low speed ABS activation goes, we have a thread for that:
Brakes - Low Speed (below 5 MPH) ABS Activation

 

Thanks for the tips, I'll try the bleeding and see if that helps first-great thread on the service bulletin...
Edit-It turns out it was a bad brake line going to one of the back wheels-the master cylinder was fine-still not sure why I couldn't find any leaking fluid, but got it all patched up and back to normal-thanks....