Last weekend I drove freshly repainted Mistress into Lexington for display at a local car exhibition, and a few blocks from the site the brake petal went clear to the floor without notice! Yikes!!! Didn't pump up at all, and thank goodness for the hand brake. When the show was over and temps had cooled down, I was able to get her the 17 mi. home with brakes acting normal? Power bled the brake lines twice, and she drove normally twice during the week. Back to Lexington yesterday, and again the petal went straight to the floor in the city traffic, way too much excitement for me! The common denominator both times was high air temperatures, and again she behaved well on the return trip home when things had cooled down. There are no leaks at the master cylinder, fresh brake fluid levels still full, so I suspect that an 8 yr. old 19mm MC has failed, on a 1973 in otherwise excellent condition with 54K miles. What else does this 914 brain trust group think could be the problem? Should I just go ahead and order a new MC?

had this happen to me last year and that lead to replacing all 4 calipers, lines, mc etc. but it was the MC , but also had a bit of a leak, AND rusted rear calipers, oh, and my hand brake cable on one side was so rusted there was zero action!
-have fun replacing the MC they are a pain! sounds like since your hand brake was working your cables and calipers should be ok. I ended up replacing the cables and lines too- you might want to atleast replace your hoses while you are at it.
Phil

Have already replaced hoses, pads, calipers are fully functional, just caught me by surprise when I thought everything had been covered!

So on a scale of 1-10, what was the "pucker factor"?

I think the answer is obvious......Don't drive into Lexington.

When the brakes fail, you enter what AC/DC front man Brian Johnson once described as "The Bunny Zone".

The Bunny Zone is where you are so scared that your rectum starts to quiver and twitch like a bunny rabbits nose.

I have been in "The Bunny Zone" multiple times in the past. Mostly racing where it was driver induced. I never want to go there due to brake failure.


On topic: check to make sure the proportioning valve is bled properly. Sometimes air gets in it and causes issues.

That sounds exciting to me!

However that is not supposed to ever happen.
I have witnessed and helped troubleshoot a similar problem on a TR4 however when the pedal went to the floor it was entering the fastest corner on the track at ~100 MPH :-0

With our dual redundant inline master cylinder, you should have the second circuit still working if the first fails.

For the pedal to go to the floor with no ability to pump it back up, it sounds like you already have one failed circuit, and the second circuit is in the process of failing.

Many years ago my sister had a similar problem (on a Plymouth Horizon...). It turned out an exhaust leak was blowing on one of the brake hoses softening it up. She would step on the pedal and it would go to the floor when the hose ballooned. In her case, she could pump the pedal and the fluid would cool the line and solidify it so the brakes would work for a while until the heat built up again.

Harvey

Bad master cylinder, guaranteed.

The Cap'n

I would listen to the Krusty One!!!


Vysoc

I am surprised no one has suggested cleaning the front seat...................


while you are there.

Or...a problem like I had. My front left caliper was dragging and causing the brake fluid to boil. Pedal goes straight to the floor when that happens. Once I let it cool down the brakes returned.

Drive the car a couple of miles and feel your rims. If one of the rims is very hot you are boiling your brake fluid.

Otherwise, the Cap'n is probably right

Thanks Cap'n, my assumption as well, and ordered a replacement this AM.

I have had that exact problem.
It would only happen on long drives where the front caliper would get hot enough.