685 caseih brake problem

[cfarmer]

I can bleed brakes and get good firm petal brakes will work fine while running but after it sets over night they will be gone until they are bled again. whats going on ? i have rebuilt both master cylinders
 

[RickB]

Is the standpipe in the "keep fill" tank broken or pulled out? If not, it's time to pull the axle housings and replace the piston seals and maybe the brake discs as well.