While my brake problem is on my Mark 2, I am posting it on the E Type forum as it is much more active than the others and the problem could apply to the E types as well. Recently while driving my MK 2 I felt the brakes self applying without any contact on the brake pedal. It got so bad I had to drop to second gear to even pull off the road. After a few minutes of pondering and examination of everything I could see, with the engine off, I could roll the car and took off. After about seventy miles it happened again, and another twenty miles a third time. Each time after five or so minutes the brakes would free up and I could proceed. A Jag friend suggested that it could be a vacuum leak which would cause an imbalance in the pressure equalization on the two sides of the booster diaphram, which would cause the atmospheric side to push and apply the brakes. What has me baffled is how I was able to drive these distances after they freed up before it happened again. Has anyone experienced this problem with a vacuum boosted system who might be able to suggest where I start and what the likely cause might be?

Private Restorer.66' 3.8 Mark 2, 67' Ser. 1 FHC "E"

Submitted by bonnettoboot@e… on Sat, 06/19/2004 - 17:57

With any luck it may just be that there is insufficient free play at the pedal, if this is the case, as the fluid heats up and expands it backs up and self applies. But I think you'll find the problem is in the hydraulic system, the valves which are supposed to allow the pressurised fluid to return to the reservoir are not functioning! strip and rebuild!

Jaguar affectionado and etc.

Submitted by jklein@genphys… on Fri, 06/18/2004 - 08:46

George,

My suggestion only makes sense if you are only having one wheel lockup. If it is all of them then I'd look elsewhere. When it happened to me, the blockage was not so complete that the pressure at the caliper was equal to normal pedal pressure so some , but not all, pressure was relieved. For that reason the caliper drags but doesn't clamp initially. The dragging produces heat which causes everything to expand and lock up. After sitting it cools off and down the road you go until it heats up again.

BTW, if it is this I also suggest changing out all of the lines, not just the collapsed line as they should all be the same age? Also, as lines are not horribly expensive, you might try this first and see if it solves the problem.

Jeff Klein
1970 OTS, BRG

Submitted by georgejones123… on Thu, 06/17/2004 - 18:07

Thanks Jeff for your suggestion that it might be a collapsed brake line, but what has me completely baffled is why I can drive many miles (like 20 or 30) after it has sat and the brakes free up before it happens again. If it were a collapsed brake line I suspect that it would happen every time the brakes are applied. After canvasing several Jag friends the consensus seems to be that it could be a leaking booster(servo)where the leak allows atmospheric presure to enter on the push side of the main diaphram which then applies the brakes. I know that when the brakes are off the air valve allows vacuum on both side of the servo diaphram to be equalized, which means that should there be a small leak on the push side, air will leak in and unbalance the pressures and put the brakes on. This theory does not explain why I can drive so many miles before it recurs, but aside from that, does make some sense.

Now that you have alerted me to the problem you had, I will check this out on my car.

Thanks for the contact.
Private Restorer.66\' 3.8 Mark 2, 67\' Ser. 1 FHC \"E\"""

Submitted by jklein@genphys… on Thu, 06/17/2004 - 16:15

When this happened on my E-type it was a collapsed brake line. They collapse on the inside, not visible from an exterior examination. Is is all wheels or just one? If just one, this is probably the culprit. You drive and apply pressure and one the corner that is collpased the fluid cannot return. This causes the brake on that wheel to drag which causes it to heat until it siezes. Letting it sit lets it cool off and then you go down the road until you apply them a few more times and it then heats up. Interestingly, the car did not pull (it was my right front) like you might think it would with just the one caliper stuck. It stopped like I was standing on the pedal. If it does it again, check to see if one coner is significantly hotter than the rest. Then take that brake line off and try to blow air through it both ways?

Good luck,

Jeff Klein
1970 OTS, BRG