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Self-applying brakes

Self-applying Brakes

Dear XK lovers,

Thanks for all your help with valve guide and shop questions, everyone. Sounds like many of you had a great time in merry old England for the 50th. I would have liked to go and combine celebrations as Land Rover also got fired up in '48. So much for small talk. I write today on behalf of my sister. She recently got her fully restored '56 XK140 DHC back in her posession after 6 long years. We had a reunion of our own as I drove to Rochester NY to take the car for a drive. She had complained of the following trouble. After 5-10 miles of stop and go, the brakes begin to apply themselves. It's as if pressure is building in the system. After a few stops, the brake lights are on all the time, and she could smell the brakes. Pulling over and resting 15-20 minutes releases the brakes and starts the cycle anew. Before going for a drive, I pulled the left front wheel, and found nothing amis. The only thing I did notice was the shoe/drum clearance was very small, but not dragging, as the shoes would not allow the drum to cock sideways even far enough to clear the grease fitting on the hub. I then took the car out for a short drive. When the car is cold, brake pedal travel is barely perceptible, almost no pedal movement to fully apply the brakes. After 5 or 6 miles and as many stop signs, I could feel resistance from the brakes. Simply depressing the clutch pedal would halt the car from 30 MPH in two hundred feet or so (from memory, no measurements). There is no pull to one side or the other, and all four wheels seem evenly (but excessively) hot upon stopping (the car has wire wheels). Just driving the car was such a thrill that most of my troubleshooting sense went out of my head. I should have removed the reservoir cap to see if this would release the brakes (excessively full reservoir with plugged vent not allowing for fluid expansion with heat?) And then maybe pulled a wheel again after cooling to see if I could force fluid from the reservoir to the wheel with the pedal, then see if return springs would pump fluid back to the reservoir. Anyway, it seems to me insufficient shoe/drum clearance (minor contributor) is combining with fluid not flowing back to the reservoir on pedal release (major contributor) to "pump up" the brakes with use/heat. Can anyone give suggestions on a likely cause? Or let me know if I'm way off base? Kathy (my sister) will take the car in this week to a local mechanic with good vintage sportscar/Jag reputation. I'm trying to help her out from long distance and give her the benefit of all your experience. Any help you can give would be greatly appreciated. (I'll post numbers on her car soon.) - Thanks again, Dave Gomes, 51 Jaguar XK120 OTS 670640 Indiana, USA

Try checking the brake hoses. - Terry

I had a similar problem in my 150 which has a brake servo behind the left wheel. I don't know if your 140 has the servo or not so this may not be of any value to you. I first used an overhaul kit on the servo. When that didn't help much, I bought the vacuum kit for it. Then, still not satisfied, I put in the air valve kit. After having done all this the brakes actually work, and how!! I would start up and drive the car with no problems. However, as soon as I touched the brakes (it didn't matter how hard), they would start to apply themselves and it became a race to see if I could get home before the brakes locked up. The answer was in the shaft that runs through the servo. There is an adjustment that must be very precise. I found the specs in a Mark 2 manual and could probably dig it out from my home if you need it. Once that was done the brakes worked reasonably well. I ultimately became dissatisfied and bought an after market servo from Terry's Jaguar for something around US$300. That has work fine. Someone on the list asked if they could drive their car without the brake servo. I tried and do not recommend it. It takes an awful lot of pressure and even in a panic, I don't think I could step on the pedal hard enough to lock the brakes. I hope this helps although if you don't have a brake servo, I can't help you. - Don Sime, 1958 150 fhc

I think too little clearance may be it. in my early days as a mech. I didn't back off the shoes on a Porsche far enough...brought him to halt 15 miles down the road. The boss wasn't pleased. I never did understand why...hot drums are supposed to exand, right? That should be more clearance. But I suppose the pressure of hot fluid outdid the expandsion. - Jim Warren XK140 dhc S817518

The brake lights being on all the time is the big clue. It means that the probem is not brake shoe adjustment, nor is it flexible hoses. It is the master cylinder or pedal adjustment. Either there is not enough free play when the pedal is released (sorry, I am not familiar enough with these to describe how much there should be), or the master cylinder is assembled wrongly, or with the wrong seals. - regards, Mike Morrin, XK150 (15 years into a 2 year restoration)

Dave, miscellaneous ramblings... besides checking the master cylinder,.. look for: 1. smashed brake line 2. some owners have complained of rubber hoses and piston cup swelling inside from using standard brake fluid with alcohol in it. You should be using Castrol LMA or Girling Crimson I think. If the car was serviced by someone not familliar with OLD British cars, they may have topped it off with a fluid that eats natural rubber and turns it to Jelly. The brake spring tension is not enough to push the piston back when everything is swollen. 3. Some Brake Master Cylinders are close to the exhaust manifold and they can get very hot if the protective material is missing from the master cylinder heat shield. this may explain the problem occurring when hot. - e. blake

Dear Dave, These sound like the classic symptoms of failure of the flexible brake hoses. The hose compound breaks-up internally and while brake fluid can be forced down through the hose by applications of the pedal, the passage of fluid back up the hose is blocked so the pressure at the slave cylinders increases each time. Pulling over and waiting will allow the fluid pressure throughout the system to even up again. I would immediately replace all flexible brake hoses on the car - not such a bad idea after a six year lay-up anyway. Note: you can't always find the rotten hose through visual inspection, and taking this simple precaution will help preserve other more expensive components. - regards Jon Garde.

Thanks for your replies. All are in the hands of my sis. A pint to the winner. I'll keep you posted. (Much more brief than the last post, eh?) - Dave Gomes

If you have any questions or comments send e-mail to: ted@jag-lovers.org
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