Having an issue with my 1998 E350 Box Van's Brakes. I recently had Both my Brake Light and ABS Light come on at the same time. Brake Pedal became Very weak and felt as if just the front brakes were working. Found 1 rear wheel cylinder leaking so did a full rear brake job to both sides including new cylinders. This help a little but not much and both lights are still on. Changed out Master cylinder with no change. Bled brakes all around numerous times and no change.
now when I turn the key to the ON position I hear 2 clicks from under the hood and the light shut off for a second when I start the rig but come back on after about 5-10 Seconds..

This Gremlin is getting me... What or where should I check next... ANY info. Is Greatly Appreciated!!!

Has Rear Drums and Disk Fronts...

 

You have 2-wheel or 4-wheel abs? Have you ever cleaned or replaced the abs speed sensors? The rear one at differential is not difficult to DIY. Once done, try to reset it or disconnect the battery cables (both) for 15 minutes or so.

 

Aurora ,
Your vehicle has a pressure differential valve in the hydraulic brake system.
When pressure in one part of the system dropped due to the leak then the valve moved to an off center position which is usually centered is now moved to an off center position.To move it back and make the light go out just open a bleeder screw on the other side of the hydraulic system and with the key on , bleeder open gently depress the brake pedal until the light goes out and STOP THERE.and close the open bleeder screw.Release the brake pedal and all should be back to normal.