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check the brake booster if the pedal is really hard and no feel to the brakes check the hose to the booster from the engine try pumping the brakes does this effect the idle of the engine?? it shouldn't
try bleeding your ABS block first, if that doesn not work could need a master or proportioning vavle. Some scan tools are capable of bleeding the ABS system on some vehicles, and some you have to bleed manually.
suggest you "look" for fluid loss issue. fluid has to go somewhere right? either a bad rear wheel cylinder.which inside of rear tires will be wet with brake fluid..either passenger side or drivers side. or a bad brake caliper on the front.which will tell on itself by leaking fluid onto the ground when brakes are applied..brake fluid leaking will tell on itself by inspecting the inside of each tire,on the inside of the tires,front to rear. thank you for choosing fixya.com.
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.
You may have a leak in the booster or master cylinder. You can put a pressure tester on the system to test for leaks. It does sound like you have an air leak. You can try to bleed the master cylinder, then slave cylinder, then brake lines.
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.