So it's been about two weeks since the mysterious ECU issue seems to have gone. Fingers crossed it was just a loose connection. Like Bugsymike said, it would be nice to confirm that was it and find the connection wire!
Today, another mystery. Driving along all is well. as I get home and turn into my driveway, NO BRAKES! WTF!!! Pressing the brake pedal all i hear is a kind of venting? I was able too grab my hand brake in time! I looked under the hood, brake fluid level is good. Checked under the car for leaks, nada!
I pressed the pedal and again it went soft and had that vent sound. I also noticed that the the engine would drop in rpm when the pedal was pressed.
I had replaced the master cylinder a few years ago and did a full pads, rotors, caliper job as well.
Not sure what this is all about?
I would guess a hydraulic failure, either the pressure seals in the master cylinder (unlikely to be the end seal by pedal rod as you would lose brake fluid down the pedal) or if your Spider is like the 1971 Alfa Romeo 1750 GTV I had many years ago the brake servo, that had two separate brake servos, one for each section of the dual circuit braking system mounted independently under the bonnet/hood.
Either of these can result in a sudden loss of braking with no loss of fluid.
I would say frightening though it is , due to "dual circuit type systems, though the pedal may feel like it drops to the floor, in most cases it drops approx 50-70% which though not good should mean the other half of the circuit has some stopping power.
Unlike a 1955 Land Rover Recovery vehicle I drove to a breakdown for the garage I was an apprentice for in 1970, I had already gone down a very steep hill at usual "mechanic" speed got to the customer broken down car and went to turn in the wide road to be in position, touched the brake pedal and totally nothing, I shot up a high kerb and a grassy bank! Fortunately no damage, but totally no brakes either, a pipe had split and as that brake system being old only had a single pipe from the master cylinder , so any issue resulted in total brake failure, so I drove back on the hand brake.

By the way if a pressure seal in the master cylinder, much the same in a clutch master cylinder, if you press the pedal slowly it fails, but if you smack the pedal down it has the affect of opening the seal better to give a proper reaction. I only mention this for diagnosis, not as a suggestion for continued use.

If the brake servo vacuum side fails it usually results in a hard pedal with little travel, but requiring a lot of pedal effort top stop the vehicle.


