Odd brake problem on an Arntz/Butler.

[RallySnake]

I built my Arntz Cobra with Jaguar rear brakes from a XJ6. It took a little machine work on the mounting bosses to get them lined up on the XKE differential bracket correctly. On a recommendation from the Powermaster Brake people, I used a stock 1 inch single master cylinder and a remote vacuum booster. The front brakes are a combination of Chevy Monte Carlo rotors and Dodge Dart calipers as recommended by Butler Racing.

The car brakes very well for a few thousand miles. The problem is that after the rear brake pads wear down about one eighth of an inch, they simply quit working. I will find myself pushing much harder on the brake pedal only to make the front tires start sliding. I have bled the system with no effect. I have added a 2 pound residual pressure valve to the system at the master cylinder. But somehow, When the pads wear down to the same point, the rear brakes give up the ghost. There are no leaks anywhere.

I used a quarter inch brake line from the master cylinder to the brake booster and a quarter inch line from the brake booster to a 4 port junction. From there, I used three 3/16 inch lines, one to each front wheel and one to the rear. At the rear end there is a 3 way junction with 3/16 lines going to the calipers. I am thinking of replacing the line to the rear wheels with a larger one, but theoretically, that doesn't make sense. The pedal does not go to the floor and feels just fine even with worn pads. Replacing the rear pads always solves the problem for awhile. But my collection of slightly used rear pads is getting pretty large!

Any suggestions to solve this problem would be appreciated.

[Bob Putnam]

RS,

You're right - it doesn't make sense.

Larger lines will have no effect.

If you open the bleeder fitting, can you push the brake pedal go all the way to the floor?

Frankly, I think it's the front pads getting glazed. The rear only contributes about 30% of the braking. I realize that you haven't been replacing the front pads, but are you "breaking in" or testing the rears after new pads, possibly deglazing the fronts?

[Dirt Cowboy]

Are the calipers and rotors matched? I don't know much about Jag stuff, but on Mustangs, the Cobra rear brakes uses a thinner material pad because the rotor is thicker. I'm curious if that priciple would work inversely on Jag stuff? I.E., if you had a caliper for a 1.25" rotor and a .81" rotor disc. Does that make sense? When you install new pads in the rear, how much clearance is there 'twixt the new pads and the rotor? Just asking.

[Mike Braddock]

Have you taken a good look at your master cylinder linkage? Maybe the pushrod is somehow binding causing the increased pedal force? Greg has a good thought about piston travel on the calipers as well. Have you removed any of the calipers to check this?

Mike

[Dirt Cowboy]

Thanks to Mike, piston travel was exactly what I was trying to get out. Good thing it's Friday. I need a weekend.

[Richard Hudgins]

Rallysnake,

I am assuming that you have a split circuit master cylinder with front and rear ports.

You may be having a problem with the internal pistons on the MC. If the internal fill port for the rear unit is covered by the piston, it will not fill and will bottom in the bore. This would give you front brakes and no rear.

As to caliper travel, the piston in the caliper will just keep being pushed out by the fluid pressure. It does not matter how thick or thin your rotors are. There is nothing to stop the piston from traveling all the way out of the bore. (Except for the pad and rotor being in the way.)

I would check the MC travel by opening a rear bleed nipple and depress the pedal. If the hardness is there, the problem is internal. If this isn't it, I do not have a clue.

Hope this helps.

[RallySnake]

Thank you guys for the replys and ideas.

Bob: Yes, if I open the bleeder fitting on any of the rear (or front) calipers the pedal will go down to the floor. After replacing the rear pads, there is a break-in period before the new pads are fully seated and become fully effective. I can really feel this when braking hard on mountain roads on a rally! I only replace the front pads when a new technology comes along because they wear so slowly. They are huge compared to brakes for other cars that weigh 2450 pounds.

Greg: The XKE rotors are about 5/16" thick (I haven't learned to think metric yet) and the XK6 rotors are about 3/8". I used the XKE one because the offset matched the caliper better. It's unlikely that the small difference in thickness would matter much.

Mike: The M/C linkage is very simple. I used a pedal from a Vega (my doner car) and a rod with a clevis pin into the M/C. The pedal travel doesn't change much unless a bleed valve is open. Then it goes to the floor. I have not removed the rear calipers since rebuilding them about 5 years ago. They are REALLY hard to get to. You have to dissamble the whole rear suspension. But you may well be correct. These calipers are very old and even though I honed the bores before assembling the pistons and new seals, there may be some kind of irregularity in there. It's is hard to understand why all four pistons would stick at the same point, though. When it quits working, there are NO rear brakes.

Richard: The M/C is a simple, single piston 1" bore GM unit as recommended by the MasterPowerBrake people. This is necessary to work with the vacuum powered remote brake booster. I know now that it is a little dangerous to depend on a single M/C, but it has worked well for over 100,000 miles. Maybe it's time to buy one of those Buick electric power master cyclinders and rebuild the whole thing. That would get rid of the big power booster unit in front of the engine and the backup electric vacuum pump under the fender. It would give the additional safety of dual circuits. I am not sure that it would solve the problem as described above, though. From your ideas, the most likely culprit(s) are the rear calipers, themselves. However, any other ideas are welcome.

[Mike Braddock]

RallySnake,

What happens if you pull the brake pedal all the way up by hand a few times and then apply the brakes? Do you still have no rear brakes? If so, it sounds like it's time to pull the rear calipers and sneak a peak . Are you sure the pads aren't hitting part of the mounting brackets preventing full piston travel??

Greg, you said it--I need a weekend too! How's the final assembly on your beast coming along?

Mike

[RallySnake]

Mike,

Thanks for your input. I will try that next time it happens.

Paul

[Curt C.]

Are the rear calipers ratchet type action? Some people have this problem on ford nine inches using Lincoln Versaille brakes and not having the e brake hooked up. The ebrake must be used occaisionly to ratchet the caliper and take up the slack. Just a thought.

[RallySnake]

Curt,

You know, there is a rachet action mechanism in those calipers! I had forgotten about it over the years. They are supposed to self-adjust, but I was under the impression that it only adjusted the hand brake pads. I will dig out the manual and see if it is supposed to adjust the main brake pads, too. I use my hand brake all the time, but he adjusters may not be working. Thanks for the idea.

Paul

[Bob Putnam]

Stock XJ rear emergency brakes are completely separate from the hydraulics.
See http://www.erareplicas.com/427man/brakes/service.htm

[Slide]

Paul, don't know if this has any thing to do with it but it sounds like you are getting excessive wear on your rear pads compared to the fronts. Have you used different brands of pad? If so is there any drag on the rear wheels when the system is warm. When they go out, can you jack up the rear and turn the tire by hand with the brakes applied?

[RallySnake]

Slide,

That's the funny thing, the rear brake pads wear at a normal rate. A new set will work fine for a year or two. But, when they lose about 1/8 inch of their thickness, (new pads are about 1/2 inch thick) they start fading, big time. It is a very dramatic change. I'll be scooting down an off ramp with my foot on the brake and suddenly I'll realize that I'm not slowing down! When I push really hard, the fronts start to lock up and slide. I have not tried to turn the wheel while someone pushes the pedal because the problem is very obvious. Replacing the pads is not too hard and I used a number of different brands. The last set were Pep Boys Semi metalic. Now I have the best Raybestos pads that I could find. It's just such a hassle to dissasemble the whole rear suspension to get to the calipers. I wish I could convert them to outboard calipers. Anyone know how to do that?

Paul

[Slide]

It sounds to me like the front/rear bias is off. Get all four wheels up and have someone gently press the brake while you turn the tires by hand. The fronts should tighten easier than the rear. If you've been using the rear for your primary stopping, then they could overheat and glaze. Good luck

[RallySnake]

Clyde,

Thanks for your reply. By brake bias being off, you mean that the rears are doing more work than they should. This is not the case. I have run a number of Solo2s where brake bias is very apearant. If you stab the brakes hard going into a tight turn, too much rear brake bias would cause the rear to slide out like when you do a handbrake turn. This is not a problem on my car and in fact it stops very well with new pads. I really surprised my navigator (Ron) on the last rally. He knew that I had been working on the brakes and asked if they were OK, before the rally. I assured him that with the new pads, they were fine for another year or so. Later, we were looking for a checkpoint with only a minute left and were behind a slower car (Suburu WRX). Ron was worried that we were going to be late to the checkpoint and indicated that I should pass the Suburu. She was doing about 60 and approaching a curve. I dropped back a little then started accelerating moderately, setting up the pass for the exit of the curve. As the following straight came into view, there was the checkpoint, 40 feet away. The Suburu driver locked 'em up and smoked the tires trying to get the speed down before hitting the checkpoint hose early. We were doing maybe 10 mph faster than the Suburu and we stopped much faster with no squeeling tires, no smoke and had time to kill before hitting the hose. I turned to Ron and asked him if the brakes were good enough for him. He was impressed.

Paul

[Jamo]

Hey Paul

Sorry--didn't notice this thread before now.

I don't know if this is helpful, but knowing the systems used by others in the same cars sometimes triggers some ideas (you might want to e-mail Monster re his Arntz).

My Butler has two seperate Lockheed master cylinders--one for the front, and one for the back--with a balance adjustment. The rears are now XJS brakes to replace the original smaller setup. The fronts are from an Imperial. I do use the emergency brake all the time when I park, and there's some kind of adjustment knob at the base of the handle that I keep fairly tight (not sure what it does--but I like to turn things thinking I know what the he!! I'm doing). No vacuum assist.

My pedal is always slightly soft at the top but then it grabs stiffly within an inch after pushing it. Just got back from the run to Beale through the Sierras, and I noticed I was able to brake a little deeper than Pat in his ERA (probably due to the SB v BB thing). No problems with wear or balance--and alot of autocrossing.

BTW, my Butler does have an access panel, but it does mean pulling the leather panel out from behind the seats. Are you SURE you don't have a panel back there?

[RallySnake]

Hey Jamo,

Good to hear from you! Outbraking Pat are you? These Arntz/Butlers have some great engineering behind them. There's a lot to be said for the front mid-engine layout. If you can get the Jag rear suspension under control and some good tires, they really do handle. On a recent birthday of my son's best friend, I took him for a ride. We happened onto a big parking lot at a mall. After some 50 mph donuts in both directions with the car all sideways in perfectly controlled drifts he had a big smile on his face. I did too!

I have been thinking of cutting an access hole in the rear bulkhead for access to the trunk. It would let me stuff jackets and hats out of the way easily. But, until now, I had not thought of cutting a lower access panel for the brakes. That's a good idea and would make pad changing much easier.

When I was building the car, I worked with a guy at Master Brake service to put my system together. His plan of using the remote brake booster is often used in heavier hotrods. It allows vacuum boost to help apply the brakes. Later, when I talked to Ron Butler, he explained how the brakes should be set up with two different sized master cylinders for front and back and no boost necessary. Someday, I will probably change over just to get the big booster unit out of my engine compartment. However, I do not think that is the cause of the basic problem. I am thinking that the pistons in the rear calipers are sticking at a certain point. That seems to be the best explaination.

Great shots with the U2! Thanks for sharing them. That must have been a fun trip. I gotta get a suit like that for rallies, the plumbing would be very useful! There is one coming up on 28 September. Check out http://carzero.freeservers.com/. It would be great to have another "supercar" out there!

Paul

[Larry Spillman]

I just sold a 1962 XKE Roadster with orginal rear brake pads in car thats 48 years of wear so if you think 2 years is normal wear for jag rears your way off. It sounds like a two fold problem limited pedal or piston travel or rotors too thin for application, over hard pads will glaze and do nothing for you.You shouldnt run race pads on the street its a bad combo. Jack car up see if you can turn rear wheels with brake applied hard if not remove wheels and start looking for the cause of restriction your 2 pound pressure valve may be too low to keep pistons close to rotors so when you apply pressure you run out of pedal travel before piston travels far enough.Did you bleed lines again?
Good Luck L Spillman

[Rick Parker]

Larry: This thread is 8 years old.