Looking at the rebuild kit their must be 5 different o-rings involved into a rebuilding of the unit ,my guess one or more are sipping air past then magically sealing back up ,nothing wrong here.It seemed after a load was put on the system is when it would act up again,that is ,after a test drive and under heavy braking the braking system lost its power brakes,rechecked the vacuum pump came back with a very low vac pressure,it had pressure but would take a longer period to reach the pressure than the pumps I tested on my other 617 diesels ,3 seconds compared to less than 1 sec. for the 2 others I tested.This includes the shut off issue that would work ,then once after heavy braking it lost its ability to shut off the fuel pump .This maybe a good time for some owners to get familiar with how important the vac system is to the braking system ,without power brakes you lose atleast 50% of your braking distance and time to respond to emergency situtations.For example ,during this issue I was coming into a parking spot and was suprised by the slow brakes and really had to stand on the brake pedal before reaching the curb stop,a real cold sweat feeling.