Had a new master cylinder fitted to 1993 rodeo but brakes are hard when you apply pressure at first but then brakes slowly loose pressure and pedal goes to floor

Did you bench bleed the master or slave cylinder ? Is Your Brake pedal Hard ? Could be your Brake booster . send image for more understanding of your problem

Sounds like your master cylinder is bad on your brake system. If you are stopped in your driveway(not on the street) leave the vehicle in park. Start the car and place your foot on the brake with light pressure and keep this pressure on the pedal. If the pedal continues to drop slowly to the floorboard your master cylinder is leaking past the primary piston in the master cylinder. If this is the case have this repaired immediately. Your brakes could just all of a sudden not be there the next time you apply them when you need to stop.

if your brake master cylinder has four separate lines to feed all four brake components separably check the out put at that outlet by cracking the fitting loose and applying brake pedal pressure slowly if no brake fluid (Dot 3 or Dot 4) comes out then you need a new master cylinder if there's only two lines and your dealing with only one brake cylinder not getting fluid then its your pa-portioning valve usually located down stream from the brake master on the fire wall

you have to bleed each wheel at the caliper, starting with the farthest one away from the master cylinder

The master cylinder in your Dodge Caravan is responsible for forcing fluid pressure to the four wheels when you apply the brakes. The master cylinder is also the main storage vessel for brake fluid. If the seals inside your master cylinder develop leaks, you will lose brake pressure internally. This could cause problems for the brake booster, as well as causing the brake pedal to sink to the floor when you press the pedal. The master cylinder can be replaced in about 30 minutes.

• 1 Press the brake pedal a few times until you have a firm pedal.
  
• 2 Open the hood and prop it up with the hood support rod. Locate the master cylinder, which is mounted to the brake booster on the bulkhead between the engine and passenger compartments, just in front of the driver. Spray the top area of the master cylinder liberally with brake cleaner. Push down lightly on the master cylinder filler tube and turn it counterclockwise to remove it.
  
• 3 Locate the fluid-level sensor connector(s). There may be one or two sensors on your vehicle and they appear as wires, in a plastic housing, plugged into the side of the master cylinder. Remove each connector by depressing the locking clip and pulling straight out from the master cylinder.
  
• 4 Remove the brake lines from the master cylinder by turning the fittings counterclockwise with the flare nut wrench. Gently pull the brake lines away from the master cylinder.
  
• 5 Remove the bolts securing the master cylinder to the brake booster by turning the nuts counterclockwise with a socket and ratchet. Pull the master cylinder straight out, being careful to avoid spilling brake fluid on your vehicle's painted surfaces.
  
• 6 Verify that the rubber O-ring seal is in place on the new master cylinder and then slide the master cylinder in place over the mounting studs. Thread the nuts onto the studs by turning them clockwise. Tighten the nuts to 18 foot-pounds. You may have to hold the booster actuator rod in position as you install the master cylinder.
  
• 7 Fill the master cylinder with new brake fluid and thread the bleeder fittings into the brake line ports by turning them clockwise by hand. Install the rubber tubes onto the fittings and secure the tube ends in the master cylinder filler opening. Make sure the ends of the tubes are completely submerged.
  
• 8 Depress the brake pedal a number of times, slowly and evenly, while a helper watches to ensure that no more air escapes from the bleeder tubes. Remove the bleeder fittings by turning them counterclockwise and carefully set them aside. Reconnect the electrical fittings by firmly pushing straight in until they click.
  
• 9 Thread the brake lines into the proper ports by turning them clockwise by hand. Tighten the fittings to approximately 13 foot-pounds with the flare nut wrench. Have your helper press the brake pedal slowly and evenly five times and then hold it. Slowly crack open the front fitting by turning it a quarter-turn with the flare nut wrench and allow the brake pedal to fall to the floor. Have your helper hold the pedal on the floor as you tighten the fitting. Repeat this process until no more air bubbles appear. Repeat this process on the rear port.
  Retighten the fittings to 13 foot-pounds. Install the filler tube by placing it on the filler opening and turning it clockwise. Refill the master cylinder to the "Fill" mark or 1/4-inch from the top
  

perhaps the master cylinder is sick put a little pressure on the pedal and keep your foot on the pedal just to keep it hard a little if the pedal goes down slowly the it s the master

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.

Firstly it might be an idea to release the brakes so you can get the vehicle to a suitable place for repair. Something has gone wrong in the brake master cylinder or the pressure valve (if fitted) although I suspect the former. The hydraulic pressure can be released by loosening the nut on the brake pipe that goes to the rear brakes or undoing the bleed nipples on the rear cylinders a couple of turns. Try to bleed the brakes enough to get them working and then drive to a place of repair. Otherwise remove the master cylinder, strip down and repair or replace.

Remove the old master cylinder. Use line wrenches to avoid damage to the brake line fittings. Bench bleed the the new master cylinder you have purchased. The parts store will tell you how to accomplish this and sell you the necessary bleed kit (note it may be included with the master cyl.). Carefully install the new master cylinder. Reconnect the brake lines and do not over tighten. Now with a helper, bleed the brake system. Start at the Right Rear wheel. Find the bleeder located on the back side of the wheel at the wheel cylinder. It should have a small rubber cap. Remove the cap and using a boxed in wrench, loosen the bleeder valve slightly while your helper is applying steady pressure to the brake pedal. The pedal will go to the floor and you will see and hear air bubbles come for the bleeder. Tighten the valve and then tell the helper to release the brake pedal and do it all over again. Keep doing this until no air appears. Periodically check the master cylinder and do not allow the fluid level to get too low as it will **** more air into the system. Remind you helper to never release the brake pedal until you give the OK. If he releases the pedal while the valve is open, air will be drawn into the system. Now after bleeding the RR wheel, move to the LR, then RF, and finish with the LF.

your master cylinder is gone. when stopped apply pedal and hold let up slightly then reapply if pedal sinks to floor master is bad.