When driving, the brake pedal is spongy and slowly goes to the floor

when you bleed the master cylinder do you have equal pressure from both the fittings?

Yes, I bench bled the new master cylinder and had equal pressure from both fittings. But this brake problem was there before I put in the new master cylinder, therefore it has to be something else. One day my brakes were working fine, the next day they were as I described. They pump up nice and tight when bleeding the brake lines, but get spongy and go to the floor slowly when the engine is running and it sure doesn't come to stop without pumping the brake pedal.

start it up and then push the brake pedal down then bleed the two break lines . on the master cylinder. Do they have equal pressure?

Good evening.

You need to pressure bleed the system. The van is too long to bleed with the foot method. Air may still be trapped in the system causing the pedal to go down.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

To be sure it is not a caliper or cylinder, clamp off the rubber hoses with vice grips. thensee if you have a pedal. If you do, then the issue is at one of the wheels.

If it does not you have air in the system or a bad new master cylinder.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Roy