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Old 04-10-2002, 04:03 PM
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Post My 4 year Nemisis..this aint no Rod

The recent discussion about Dot 5 brake fluid has me thinking about a problem I have had.

One of my favorite toys is a fully restored 1960 Porsche 356B Super 90. It has massive finned aluminum brake drums with cast iron liners and will stop the car on a dime. In front there are dual single action slave cylinders and in the back there is one per wheel. The master cyl is a single fruit jar type feeding both front and back. The master is a new unit and all six wheel cylinders are rehoned and have new kits in them. All brake drum surfaces have been freshly rounded and honed.

My problem is that after the car has sat for more than a day or two, one of the front brakes violently wants to lock up at very nominal pedal pressure. If I push down really hard on the brake pedal several times, the problem will go away until it sits again. I have rebuilt the front cylinders at least twice and the problem prevails and I have re-bled the system several times. I have not been able to locate even the slightest drop of brake fluid leakage onto the drum surfaces. I have taken it to Porsche Specialists who have done the same and not eliminated the problem. I think you will have to be outside the box on this one. Give me some ideas please.

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Old 04-11-2002, 07:36 PM
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I had the identical same problem with my Corvette. After a lot of trouble shooting it turned out to be a partially bad flex hose that was going to the front caliper. The hose looked good from the outside, but inside the lining had gone bad over time, and when ever it felt like it would close off the brake fluid going to the caliper. If your car is pulling to the left, that means your left hand side brake is working fine, and you would need to look at the right hand side, or visa versa. If both flex hoses are about the same age, I would replace both of them and then reblead the system.

Let us know what you find out.
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Old 04-11-2002, 08:10 PM
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After making that first post I thought of a couple other things you can try that could possibly be the problem. As usual I would try the cheapest ideas first.

On another car I had a similar thing happen, only instead of one it was two wheels locking up. It turned out that the brake peddel was not coming back far enough. It is supposed to come all the way back plus about 1/8 inch to be sure there is no pressure building up in the lines. If the piston inside the master cylinder is not allowed to go all the way back, when you take your foot off the brake pedal, you will have pressure build up in the line, each time you apply the brakes. Something for you to check that will not cost you anything.

The last idea is that if it only happens after the car is stationary for a few days, it tends to make me believe something is freezing up in the front slave cylinders. I will bet it is the same brake grabbing every time you have the problem. If you try everything else and nothing works, I would replace the slave cylinders on the oposite wheel that is grabbing. (If left front is grabbing, replace the slave cylinders on the right.)

The car is being pulled to one side because that is the side the brake is working on. Its either partially stuck or not working at all on the other side, until you work it free with the hard pressure on the pedal.

Hope that gives you some ideas to check out.
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Old 04-11-2002, 08:40 PM
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Drum brakes are typically of the self energizing type, what that means is that upon application the brake shoes will have a tencency to draw themselves into engagement. This property is beneficial in that it allows for higher braking forces with less pedal effort as anyone could tell you who has tried to stop their old soapbox go-cart with a stick rubbing against a wheel. The pivots of the shoes and diameters of drums are all closely designed to make best use of this property, if one or the other is slightly out of spec it can cause grabbing brakes.

If your drums were machined there will be a max dia spec that corresponds to the absolute limit beyond which the brakes will actually energize themselves such that they will apply more braking force than you intend. Worn components will reduce your effective max tolerances and cause the problem you describe.

I do not beleive in machining drums or rotors for this very reason, I replace them if they are worn. Shoe pivots are usually impossible to replace and will require changing out the backing plate to fix. If the pivot pins or any other component is not perfectly square with the drum or backing plate, that is another reason to change them out. Brake shoe wear will only make the problem worse.

Am I nuts? if you have driven a new car and felt how nice the brakes work you will know why I do it this way. I look at it like this, the braking system is the last place I want the tolerances to be on the edge and machining them only gets you closer to the edge. Besides doing it myself lets me completely replace all the wearing components for the price of a cheapo "brake mart" type overhaul. When replacing drums don't go for the cheapo "Mexico" drums either, they typically have a poor quality cast iron that is full of hard spots and casting imperfections. Hard spots will also cause grabbing brakes.

I buy factory or US made cast iron only. German made drums used to be top notch but recently I have seen drums that had "made in Germany" cast into the drum but only the mold was made there with the actual iron metallurgy and pouring taking place in Malaysia or other third world country, these are really crappy. Let their price be your guide.

Drum diameter is especially important because as you machine the drums the shoes get farther and farther away from the shoes which only enhances the energizing effect due to the pivot location not moving to compensate.

The reason why they do this only when cold is because the brake pad material has a higher coefficient of friction at this temperature and the slight haze of corrosion that occurs after sitting for a while only makes it worse.

I would be willing to bet that a drum replacement would solve your problem, but you would have to replace the shoes to suit. At one time machining the shoes to fit the ID of the drum was common as was adjustable pivot pins. Modern cars have done away with all these things in exchange for limit sizes on the wearing parts. Only problem is how many brake shops do you know that actually replace the backing plates when they are worn?

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Old 04-15-2002, 02:34 PM
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Thanks for good input. I don't think the problem is in a collapsing brake line. They are both fresh. I have swapped the drums left to right and front to rear and it is always the same wheel that grabs, so I am going to discount that the drum is causing the problem. In any event, the drums on this car are about $1000 each if you can find them so I am hoping that it is some other gremlin. I replaced the shoes with new ones but it exhibited the problem I am having with both used and new shoes.

I am going to try swapping the slave cylinders around to see if I can determine if it is a slave problem and then which one. I can have these rebuilt with stainless steel sleeves for about $75 each if it comes to that. I am willing to consider that the master cylinder plunger is not fully releasing and will check that, but I think I have been through that consideration before.

I really appreciate your help. You guys know your brakes.
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