On the first day of reaching 100 degrees here in Phoenix yesterday, my '61 Mk2 seem to be performing well as I drove down the freeway until I began to hear a new noise somewhat like a high pitched whine and barely discernible. Next, the motor began to act like the a/c compressor was cycling in and out. A bit later the car slowed down and began to lose power as if the brakes were being applied. Following this, the brake pedal became hard and the car continued to slow down. Finally, as I pulled to a stop in a parking lot, all pedal pressure ceased with the pedal going all the way to the floor. and back. Parking on a slight incline, I put the car in neutral and again noticed the brakes seemed locked as the car failed to roll down the slight slope. I then shut the motor off while pumping the flaccid brake pedal and the car started slowly rolling back until I applied the emergency brake. Emerging from the car, I immediately checked all four wheels for signs of brake fluid puddling. None was seen. Raising the hood, I again looked for leaks beginning with checking the master cylinder reservoir - It was full. The brake servo was next and again no leaks were observed. Recalling earlier remedies for servo issues, I banged on it gently with a hammer. It didn’t help anything but I felt better and my distressed mental state improved briefly. When reality returned, I checked all around the engine bay and spied a bit of moisture near the brake light pressure switch but nothing else. I then calmly waited for the AAA tow truck to arrive. Looking anew this morning for solutions to the brakes, I spotted a fair amount of brake fluid underneath the front of the car. Raising the hood, I determined that the leak was coming from the brake light pressure switch. The master cylinder reservoir reflected a corresponding significant drop in fluid. Likewise, paint was removed from everywhere the fluid touched.
So, does anyone have thoughts on what caused this? I should note this car was fully restored 5 years ago with new everything including vacuum servo, master cylinder, wheel cylinders, brake lines, etc and driven weekly around the neighborhood when not resting in the garage. Formally a business major, I could hypothesize a solution if I had just one of the above enumerated issues such as no pedal pressure. Or the pedal is stiff. However, I suppose it.s possible both the master cylinder and the vacuum servo are failing at the same time. After all, it’s a Jaguar!
I lived through this exact experience early in March (about 37 degrees C)! At the end of February I bought our 1963 Mk2 at an Auction in Melbourne intending to drive it home to Sydney - a distance of about 900km. I was motoring along the freeway on my way out of Melbourne city and had the same experience: car slowing; brakes binding (with smoke from rear); and rock hard pedal. I pulled over and a sharp eyed freeway road service guy picked me up and dropped me to a safe location - no charge. I had a similar experience in my e type so had some idea of the problem. In short the ambient temperature, hot driving conditions swell the fluid pushing back on the piston and the pedal but there is insufficient clearance for the fluid to vent back to the reservoir. Heat an pressure push harder on the wheel cylinders and you finish up where we did. As it happens I was travelling with a “mother ship” and I gave him a call (himself travelling a 1972 Escort twin cam per same auction). He told me to release the master cylinder bolts two turns, this cleared the vent hole and I drove home in the continuing heat without event - stoppng each now and then to make sure the bolts were maintaining their position. On an E type it’s a matter of setting an “expansion clearance” at the master cylinders (two off). The Mk2 does not have the same amenity so I finished up making up an aluminium spacer to maintain the clearance effected by the loosened bolts. There may be other ways. Could be a small difference in the master cylinders: after market and original? Hope this helps, Paul
In my experience (with four British cars) the currently available pressure type stop lamp switches are notorious for failures. Usually the symptoms is that they fail to turn on the stop lamps but they could also leak. I have since changed over to the more expensive ones sold by Ron Francis Wiring and had much better service life.
Another thing to check, although unlikely on your car with new components, is the brake hoses. As they age they can develop internal deterioration that causes in effect a one way valve preventing the fluid from returning to the master cylinder. On a Mark II the rear one could develop this issue causing both rear brakes to drag.
And finally a sticking slave cylinder in the booster (or master cylinder) could cause the brakes to stay on until the pressure slowly bleeds back to the master cylinder and to the reservoir. .