Herve and Chris, refer to Ruedi’s post above (post #5) and click on the link “this page” and read it carefully, especially the paragraphs under the diagram because I think it shows the solution to the problem you are having. It reads, “sometimes the piston can jam…” etc. They are referring to the large piston in the vacuum chamber, which is pushed back by the huge coil spring. In 2003, when finishing the brake system on my 2600 Spider, I had the same problem and, after removing the booster several times, I made a test fixture to test it on the bench. At the time, I was living in a metropolitan area of south Florida where it was very difficult to test drive my project and nearly impossible to trouble-shoot the brake system, which locked up at nearly each stop light! I was reduced to getting up at 2:00 AM and taking the car out on the roads until about 4:30 and trying to get home before the rush started at dawn. Anyway, that was my justification for going to the trouble to build this test rig. I used a vacuum pump and vacuum gauge and a high-pressure gauge reading to 4000 psi and a spare master cylinder with a long rod to simulate foot-pedal pressure. What I found is the same as others have found in the last many years and that is that the rebuild kits use a sponge rubber strip that is too large in diameter to expand the leather seal out against the vacuum chamber and it fits so tight that the huge coil spring can not return the piston when you let off the brake pedal. My solution was to slice off part of the sponge rubber expander strip with a razor, until I found the size that allowed the spring to return the piston and, at the same time, pressed hard enough that the vacuum would still allow the piston to work. I will try to post pictures of the test rig. BTW, I now live in rural north Florida, where there are nearly deserted country roads, perfect for us car nuts to test drive a car. This morning, I drove the Alfa about 20 miles, during which I twice tested the brakes from 60 mph. by braking at the threshold of lock-up; no other cars in sight!
Larry
