Brake Trouble

Please folks I need you help stopping my duster. The car in question is a 1970 Duster back half 4 link race car. The engine is a 472 stroker engine built out of a 400 low deck block. The car has disk brakes all around wildwood on the rear and aerospace on the front. We were having issues with a Strange master cylinder so we put a Mopar peice on it. now when you first get in the car there is no pedal you have to pump it up. When you start to pump it up you have to pump it and pull it back up with you foot to pump it again. Once pumped several times it seems to work fine. any insight you pentastar geniuses can provide will be greatly appreciated. thanks josh