Brake pedal drops when I start it.

[KalamazooJohnny]

Just finished new brake lines, calipers, compensator, hoses,master cylinder, bleeding. Brake pedal feels firm when car is off. When it starts, not nearly as firm. Brakes work, but not as firm as I think they should be. Still a bubble in there somewhere? Try bleeding again? Anything else I might be missing?

[So Cal Mark]

the pedal should drop when started if the power booster is working

[124JOE]

as long as it dont "slowly go to the floor"when you hold it
your ok

[TX82FIAT]

As long as you don't hear any hissing in the brake booster and there are no leaks you should be ok. Assuming the testing of the system stops the car properly.

[Eastlight]

My brake pedal drops whenever I step on it - Oh please! It's a joke!
On '80 Spider I recently installed a brake servo, brake booster, whatever you prefer to call it. What a pain in the class! Tight fit - like everything on a Fiat and my first girlfriend. First, master cylinder has to be totally removed. Then remove all intrusive vacuum hoses, detach accelerator linkage and unmount the carburetor, remove any vacuum fittings on the intake manifold (mine is not stock) that are in the way and the clutch cable sits right on top of the servo, booster or whatever. I use the terms interchangeably. That clutch cable makes life more difficult for servo/booster removal and installation, just enough to get the swearing started. After that, everything is a Mother F this and SOB that.
Worse of all, none of my manuals or schematics said anything about the mounting plate. There is a right-side up and a wrong side up. I learned that the hard way when, with wrong side up, the servo/booster yolk will not align with the brake pedal pin hole. I am happy I changed the servo/booster and thankful to a friend in NJ Fiats that generously gave me two servos/boosters he salvaged from parts cars.
After all was said and done I did not like how far the brake pedal traveled before applying the brakes. Removed the two mc mounting nuts, pull the mc out as far as possible without forcing it (about 2-inches), reach inside the servo/booster (needle nose pliers work), pull out the actuating pin.
Hold the knurled shaft (I used needle nose vise grips) then turn the cap nut counter-clockwise to extend the shaft, which will raise the brake pedal. Just do not over extend it to where your brakes are always applied. Incidentially, a 5/16 open end wrench fits the cap nut perfectly. My metric wrenches were not small enough.
My servo/booster gave no indication of failure until one day the car stalled in a parking lot. There was raw gas in the air filter pan (one of those small custom filters). Got the car home and, because the brakes did not work properly, discovered the loss of engine vacuum from the servo/booster. Removed the servo/booster hose from the device, plugged it and the car ran fine except for having to stand on the brake pedal. Now, with new front calipers and brake hoses you can push the pedal with one finger and stop the car.