Need help diagnosing brake problem

Yea i, m thinking the same but What do you think about this being the 3rd master cylinder that has blown fluid back up through the reservoir including the one that was originally on the truck when it was bought 3-weeks ago which did not look very old. We have had two different ones that we replaced in last two days do the same thing. Granted they have all been remans what are the chances of all being bad? Is there anything else that would cause the fluid to blow back up through the reservoir and not pressure up? Whjy also does the pedal feel decent until the truck is running then it goes back to being soft.

Check the break booster also, I know mine wasn't strong enough to stop 45 inch tires so I had to add an electronic trailer booster but I ran it to my brakes. Try bleeding the brakes also with the truck on see if that changes anything and if not maybe you have a leak within the lines