I have a 94 V6 Automatic 4 wheel drive. I have replaced the booster and master cylinder for the brakes and bled the system twice. The break petal goes down within a minute or two and the rear antilock break light comes on. What do I do next? Jerry

Did you properly bench bleed the master cylinder itself before installing? It must be bled or there will be air trapped inside the master cylinder itself that no amount of wheel bleeding will remove.

My first guess would be the power brake booster is leaking vac.

did you bleed all of the air from the master cylinder before you installed it, this has to be done. also when replacing the master cylinder, all 4 wheel cylinders will have to be bled, in order to get all of the air out of them. while doing the wheel cylinder bleeding , you will have to make sure that the master cylinder doe's not run out of brake fluid, check it each time you bleed a wheel cylinder , add brake fluid to the master cylinder each time or when it is getting low. hope that this has helped you.

Are you losing any Brake Fluid? Is it visible anywhere at the four wheels? Is it visible anywhere else?
The Master Cylinder may be defective. When you bled the Brakes did you maintain proper Brake Fluid Level in the Master Cylinder? If you didn't: and air entered into the Master Cylinder there maybe air within in the system in the Master Cylinder. You can bleed the Master Cylinder by removing the brake lines, adding more fluid, pumping the brakes, bleeding the air out of the system.
Are you losing Brake Fluid and can't find a noticeable leak? Then there is a good posibility that the rear seal in the Master Cylinder is leaking and the Fluid is going into the Brake Booster - which is the large wheel shaped drum located on the fire wall. In this case both the Brake Booster and the Master Cylinder will have to be removed and replaced. The Booster contains seals that Brake Fluid is very caustic to and will ruin these seals- causing future failures.
If no Brake Fluid is lost, no leak found: Check the Brake Booster Vacuum Advance. This should be connected to the outside of the Booster, attached to a hose, with the hose going to the engine. Check to see if this is working properly. Replace if needed.

leaky rear break cylinder or master cylinder

Did you bench bleed the master before you installed it???? Engine off, have someone push the brake petal down and hold it down while you crack the brake line loose at the master,then tighten it and let up on the petal.Do this several times for each of the 2 lines at the master, only cracking it when the petal is being held all the way down.Repeat...Do them separately.Have some rags below the master to mop up the mess and keep the master from running out of fluid. Then do the gravity bleed one more time on each wheel cyl. and watch for a bubble of 2.
I hope this helps you.
Yes the booster sounds fine.

Did you bench bleed the master cyl. before installing it? then when you installed the new master, did you bleed all 4 points? If yes, then check all for points for leaking caliper/s and leaking wheel cylinder/s. If the van has ABS, check the valve body for a bleeder screw and see if air bleeds out of it. If you've checked everything twice and did everything right then your new master cyl. may be defective.

5 Inspect Brake Fluid Type Brake fluid loss or contamination 10 Inspect Master Cylinder (Brake System) Worn or leaking brake master cylinder. 11 Inspect Brake Power Booster Damaged booster vacuum line/check valve, or diaphram

Most likely your vacumn power brake booster has failed. It is not rebuildable. You will need to buy a new one.