everything was normal yesterday then all of a sudden the brake light stayed on until I tapped the break peddle. They are very spongy and so I found myself using the low (automatic) gears to stop on the way home. It's sitting in the driveway now. I'm thinking master cylinder? I have read through all the other posts and found people asking if it's a 4X4? Why?

I'm not sure if the brakes are different on a 4x4 or not. I'm sure they are. I honestly think that your diagnosis could be right. First I would look under the vehicle to rule out any leaking lines, hoses, calipers, and wheel cylinders.

Check all your brake lines and whatnot, and rebleed them, if the problem comes back, you probably need to rebuild your master cylinder. Or you have a brake booster issue, but more than likely, master cyinder is what goes when you have mashed potatoe brakes.

First check to see if the master cylinder is low on fluid if it is refill it and then look for leaks , dont forget to pull the rear drums and check the wheel cylinders

changed the master cylinder with a new one (not rebuilt) and got instant relief. BUT the brake warning light comes on when I apply the brakes now. I bled the entire system under pressure three times and I have no leaks at all (clean under body). Still stumped, but happier.

The brake warning light will come on if the pedel travels too far down. Make sure the rear brakes are adjusted correctly. You should feel the brakes drag a little bit when spinning the tires.

//2000CAYukon

Wierd, my pedal travels just to about the floor and my light never comes on.

The light could be your ABS System. Uplug the eletrical connection on the combination valve by the Master cylinder if the light goes out the problem is in the normal braking system. If the light stays on the problem is in the ABS. Try jumping terminals A & H an you ALDL Connector this will give you any codes in your ABS System.

Like the previous poster stated jumper these two slots on the connecter and you will recieve any codes.http://files.automotiveforums.com/gallery/watermark.php?file=/503/178333DIN.JPG

I have no idea where to tell you what the numbers mean, but that will give you somewhere to work from.

FYI.....If you jumper A & B it will give you service engine soon light flashes, reporting any engine DTCS.

Good luck.

Thanks for all the help, It is a long road as I can't find the time (long hours at work) to put aside, but I am saving everything so it helps me fix the problems. This is the best site I have ever come across. I shall be trying my hand at fixing the problem today. I will keep everyone posted on the outcome. Thanks again.

91 Chevy K1500 4X4
"If you can't get it dirty then it's not a real truck"

I don't believe the 4WD and 2WD 1500's have different brakes, the front rotors are different. They both use the delco calipers and leading/trailing rear drums.