1996 Ram 2500 cummins
New: brake master, bled system until blue in the face
new: power brake boost unit

vacuum output: 18 to 20+ in Hg no problem, though it drops to zero as soon as the motor is shut off.

the new (reman) boost unit holds its vacuum just fine as tested by the factory service manual instructions, in line T to vac. gauge

Pedal sinks to the floor slowly .... ok .... (third) new brake master is on the way

Todays problem: tap that brake pedal three times ... then the boost is lost and then restored only very very slowly, too slow for safety in traffic.

It should restore immediately, YES??

AT the end of the booster test (to see it retains its vacuum after the motor is shut off ..... when removing the clamp from the clamped off vacuum supply line from the vacuum pump I hear a distinct hisss from the front left engine compartment. I cannot find any vacuum lines except to the cruise control under L battery and the line from the pump itself.

Is that pump meant to lose its vacuum while the motor is not running? It does and I suspect that may be related to loss of boost with a few applications of brakes

 

Youve probably got a leak somewhere in the vacuum lines. Usually happens underneath the truck in the plumbing to the 4wd stuff on the transfer case on mine.
You should see near 25" of vacuum when everything is good and sealed up.
Obviously idle speed factors into how much vacuum rebuilds after use.
I think the spec is 800rpm in drive with AC on.

I actually had a brake booster go bad on mine.. first time thats ever happened. probably rusted inside. If you can isolate it from the rest of the system you can see if its leaking. Or you can just replace it and see.

 

Unless the system holds vacuum for a long time after the engine shut down you have leak.
My trucks will hold vacuum for several days.