Brake pedal suddenly goes to the floor

[uestvega]

1981 Spider.

I was just driving and my break pedal suddenly lost pressure and the pedal travelled all the way to the floor. I still can stop so part of the brakes are working, but I am bottomed out. When the engine is off, I can build pressure in the pedal, so it feels normal, but as soon as I start the car, the pedal looses all that pressure and travels again to the floor.

I have replaced the master cylinder last year, and the rubber brake lines as well. The fluid is clear and still topped up so seems to be no leaks in the system. I imagine that the brake assist has gone.

I am thinking of removing the vacuum line to the assist to see if I can turn it into a full manual brake system to verify if assist is shot.

Any thoughts from experienced spider fixeruppers. Is it a difficult job to replace the brake assist myself any advice. I used to work at a service station years ago and can do brake jobs, tune ups, even did the timing belt, just no under the hood brake stuff.

Ciao

[azruss]

First check your vacuum line from the manifold to the brake booster. There is also a check valve going into the booster you need to look at.

[DieselSpider]

If its not leaking into the booster from the end seal of the master cylinder or at a brake line, connection or caliper seal then you may have an issue inside the master cylinder possible with one of the cups letting fluid bypass so you may just have a defective master cylinder.

The previous owner replace the master cylinder on my Spider about a year before I bought it and it was doing similar. Turned out the rubber parts inside did not react well to the silicone based brake fluid a tire shop put in it so they failed. Replacing it with a Beck and Arnley master cylinder available locally resolved that issue.