I bought a new master cylinder and tried to bleed the brakes, but two of them would not bleed while the other two bled just fine. the drivers side front and the passengers side rear would not bleed. Any ideas

You do not bleed brakes in an "X" pattern. You start bleeding at the wheel furthest from the master cylinder. Once that wheel is bled, move to the next furthest away from the master cylinder and so forth. Doing it in and "X" pattern would leave air in the lines.

A master cylinder is supposed to be bench bled first to get the air out of the primary and secondary bores in the cylinder if you didn't do this there's air in the cylinder also you may have blown the seals inside it try to bench bleed it if you feel no pressure on the master cylinder plunger after bleeding it the seals are blown and you need another one ////\\\\if you do feel the pressure after bleeding and after installing it and bleeding the lines at all wheels and there is no pedal pressure the brake booster is the problem

1. If there is a substantial fluid eruption, there are air bubbles still trapped in the system. You must repeat the bleeding procedure to remove that air. A modest disturbance in the fluid returning to the reservoir indicates a properly bled brakesystem.
   

   How To: Bleed Your Brakes - Feature - Car and Driver

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How To: Bleed Your Brakes - Feature - Car and Driver

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Car and DriverIf there is a substantial fluid eruption, there are air bubbles still trapped in the system. You must repeat the bleeding procedure to remove that air. A modest disturbance in the fluid returning to the reservoir indicates a properly bled brake system.

How to Bleed Brakes - Tips on Bleeding Brakes

Did you fully bleed the brakes after installing the new parts. It sounds as if you air bound

Still air in system, sounds like the master cylinder has ran dry during the bleeding process, pumping more air into system. The lines running to the rear brakes are long, so you must bleed the fluid out the bleed screw in the rear 10+ times each side. Bleed, bleed, and bleed some more, keep bleeding untill that pedal feels good.

I don't agree with the previous answer. I have bled many brakes on vehicles with ABS in the conventional way and so far have had no trouble.

The sequence remains the same as it always has but is usually less important today as it was before dual circuit braking - start at the closest bleeder to the master cylinder and work to the farthest.

It isn't a good idea to allow the reservoir to fully drain when changing a brake line and doing so often causes problems. In case of problems it is good to first ensure there is fluid delivery from the master cylinder by removing the troublesome brake line and use a finger to create a suction when the brake pedal is on the upstroke. if there is a good delivery there and the new pipe is clear it should now bleed normally. If it doesn't then repeat the process at the other end of the new pipe - and so on...

Make sure you bleed from rear furtherist from master cylinder and make sure when your helper is in drivers seat to pump brake pedal 4x then hold, at that holding point you open bleed screw then close once brake pedal is on the ground. Repeat 5x then move to the opp side and do the same. Then to the front furtherist from master cylinder and do the same. Making sure the brake fluid is always topped up also. Also check all your conecters in the lines are not leaking

well sounds like its still full of air youll have to gravity bleed it. if you can leave your car on a steep hill overnight with the rear end up then bleed it the do the same make your front end sit high up overnight then bleed it. air bubbles always try to make their way to the highest point

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.

Brake rubber flex line could be collapsed,,or kinked,,,i suggest loosening of the line at the master cylinder distribution block (this will give you the ability of loosening off one line at a time,,and have some one push the brake pedal down,,if you see fluid at this point then you know it has to be the line or the caliper.
if you get no pressure from the distribution block then its the master cylinders fault