working on a 90s dodge truck. having a minor problem?

[tallchic9113]

working on truck. why would this happen any thoughts Why is there brake pressure when not running but as soon as you start it there is no brake pressure on 90s dodge truck
Brakes were fine til we changed brake pads. Now there is pressure when not started and no pressure when truck is on. Why would this happen.

 

[Jessica]

its the master cylinder going bad have you brakes been felling spungy latly