Sudden total brake loss

[Landcrab]

Hello, I have a very strange behaviour on a 1966 2+2. The owner of the car told me that the car at a few occations he had lost brake preassure and he had to pump the pedal a few times. I was picking up his car and there was nothing wrong with the brakes. I changed the master cylinder the flexible hoses and the fluid.
Went out for a test drive and everything was good. Decided to drive the car back to my friend and the return trip is 20 minutes country roads and finally 10 minutes motorway. I was braking slightly before turning on the motorway, all fine and 10 minutes later when turning off there was zero brakes. There was the emergency brakes that the reaction valve sort out but absolutely nothing else and that is the last 10mm of the pedal movement. Went back to a friends house and had some coffee and 30 minutes later a few pumps on the pedal and the brakes are back!!! I spoke to the owner and he then said that the few times he had this fault was always after motorway driving.

New oil
New hoses
New master cylinder
No fluid loss in any of the 2 containers.
Absolutely no sticky pads or roll resistance at any time.

Could the vaccuum do something like this?
Or what could possibly cause this?

Is there anyone that have had this odd behaviour before?

Greatful for any suggestions

Best Regards Johan

[abowie]

Is something getting hot?

Sounds like brake fade due to brake fluid boiling.

[Landcrab]

No nothing out of the normal when I checked the brakes 30 seconds after the failure

[mgcjag]

Do you mean a loss of servo assistance brakes or a total loss of brakes.....there are very good drawings in the service manual that describe the braking system......I cant see a situation where this is connected to driving....you should be able to recreate any issues in the garage......i would be looking at the reaction valve area.........Steve

[malcolm]

I had something similar, although not as dramatic, and had a new servo unit. All fixed.

[Tommd]

Johan, based on what you describe, I would suggest the MC first, which you already addressed. Next brake fluid, which you addressed. Last the slave/servo cylinder which fixed Malcom's. My only further question would be, if it is the slave/servo, I would be expecting the one reservoir to fill and the other empty, although it would not be "required."
Tom

[Landcrab]

Thanks for the suggestions, It was a total loss of all the brakes and not just the servo assistance.
I will change the servo unit as well, but I’m wondering if no braking in combination with constant vacuum for a longer period could reverse all the caliper pistons? This of course due to some kind of failure in the servo cylinder.
It’s s really nasty fault and I have only had the same experience on a Mini with one line system so that was a no brainer.