No Brakes

[58panel]

Hi!
Been lurking and learning from all the great info on this site!
Got a problem! We installed an Chrysler new yorker front end in a project truyck! used the under floor mustang II booster and master setup. Got the metering and residual valves from Master Brakes installed.
The problem is that the system won't maintain pressure. it will pump up and work but on the initial hit it goes almost to the floor and only the rears seem to work.
This is driving us nuts!

Any suggestions?

Thanks and happy Thanksgiving!