Sudden Brake Failure!!!

[everdyl]

Well, I brought home a 72 Spider on Sunday. I haven't had much of a chance to get into her yet, but the weather was so dang nice today, I just had to take her out for a spin. Got her about 40 miles from home and went to turn off of the highway and SURPRISE! - brake pedal went all the way to the floor and the "Brake Failure" light went on when it got there.

I did manage to get her home safe. I checked the fluid levels, and they are full (but VERY dirty). There didn't appear to be any leakage from the master cylinder (if I'm looking at the right thing) or around the wheel calipers. There was a dragging sound coming from the passenger side rear, but only intermittently.

So do I start with bleeding the brakes? What could cause a sudden, massive failure of the entire braking system but leave fluid in the reservoirs? Any advice would be much appreciated. Even with the brake issue, I still have a huge grin on my face from the 100+ miles Fiona and I traveled together today!

[Danno]

If god wanted us to use brakes, then why did he invent downshifting?

[RRoller123]

I am no expert, but if you didn't lose fluid due to a leak, then it sounds like failure of the master cylinder.

[TX82FIAT]

I think RRoller ma be on target with the master Cylinder going out and possibly letting some fluid bypass the piston. In this case the brake would go to the floor. However, before buying a new master cylinder. I would recommend bleeding the brakes and purging all the old brake fluid out of the lines. This does not cost a lot and you will need to do it anyway. If you have air trapped in the brake lines the air does not compress so the pedal will go to the floor. By bleeding the system you can figure out if your master cylinder has gone out or you simply have some air in the system from the master cylinder down through the hard lines.

Did you do anything else with the braking system like adjust the acorn nut?

[everdyl]

Thank you for the advice. After a little bit of research, I do believe that the master cylinder could very likely be the problem. But I'll go ahead and bleed the system first to see where I'm at.

I hate to admit it, but I don't know what the acorn nut is or how to adjust it.

[narfire]

You'll see the acorn nut on the end of the rod from the brake booster. If you have to re/re the MC, you'll see the nut when the MC is off.
There is a picture on IAP's tech section (brakes) on how to adjust it if you have to replace the MC.

[MrWampus]

You left one clue that you should invstigate, because I had a similar experience. You mentioned a slight dragging sound coming from the passenger side rear. Check closely to make sure you don't have something wrapped around that part of the rear axle. I got a piece of string wrapped around mine a few months ago. It wrapped around the handbreak connection pulling the piston closed on that caliper. As I drove, I heated up the brake line and boiled the fluid in the line which caused my pedal to go suddenly soft. I unwrapped the string and everything was good as new. Check the other items as suggested but check to see if your handbrake connection is pulling a caliper closed somehow.