This car sat up for a few months. After my son got the engine assembled, he hit the streets. When braking from, say 45, the car shakes really hard in the front end. The tires are new, so that's a variable gone. I have not asked him about the condition of the rotors, but I assume they are ok. Would the flexible brake lines cause this?

No...all cars have flexible lines installed somewhere. My guess is warped rotors or froxen calipers.

Ed

I'd say yank the wheels and have a look at the rotors. Over time (doesn't take long), the rotors can begin to rust over, causing an uneven surface which can cause the shimmy your son feels.

I just returned from his home. The rotors are rusted a bit. I guess Dad will spring for new rotors and calipers. Thanks for the help guys!

Be sure to check your wheel bearings as well. Worn bearings will allow a rotor to warp and/or a warped rotor will help the bearings go bad.

The car shakes after braking??

yes.............until the car stops completly.

Oh, so the entire time you are actually applying the brake pedal the car shakes.

Does it shake with the speed of the car (i.e as the car slows down, the speed of the shake/shimmy slows down)? If yes, i would look to rotors, bearings, etc.

If the vibration is more in tune with the engine, then I would put suspect on something more like a vacuum leak when the brakes are applied.

Mine did the same thing when a steering box bolt was broken. Verry scary! Check for that as well as cracks in the frame around the box. Check the whole front end. There is play in something, centre liink, ball joints, tie rod ends, wheel bearings ect.

if when you apply the brakes the car shakes bad in the front id say more than likely it is the rotors being worped.

As the origional poster stated, the car was still shaking after taking his foot off the brake. Sort of a nasty speed wobble. Same thing mine did.

Warped rotors will not shake the car. Pad deposits, contaminated pads, worn or broken suspension components, bound-up calipers, or damaged bearings may but not rotors.

Surely you didn't actually mean that? Did you read what you typed before you clicked "Submit"?

Warped rotors (or otherwise uneven rotors, such as pads that sat in one place for a long time and made a particularly rusty spot on the rotor surface) are the #1 cause of brake vibrations. At least on my planet. Maybe not on other planets, but that's how it works here on Earth.

The primary causes of brake vibrations is poor machining, disc thickness variation, and worn or damaged mounting hardware, not lateral runout (warp). All rotors have lateral runout, I don't care if it's .00001" or .01", and all brake systems are designed to compensate for it. If a rotor has a lateral runout of .1" or more, something else caused it. It's an effect, not a cause of vibration.

Here on Earth, the primary causes of brake vibration, are heat-related. Occasionally one comes across a poorly machined part, and if so, one repairs or discards it at the time of installation. However, in the other 95% or more of all cases of brake vibes observed in the wild, nearly all begin occurring after a period of vibration-free performance (indicating proper machining, etc.). The vibration appears only after damage to the rotor during operation as oppposed to during manufacture or repair. The cause is rotor warpage or other damage due to excessive use of the brakes overheating the rotor; or poor metallurgy (cheaply made parts) leading to uneven dimensional changes over large numbers of heat cycles in the part's lifetime, or stress relief in the material's crystalline structure from heat cycling; or sudden immersion in water while hot, causing rapid cooling, and resultant distortion of the metal.

Many instances of heat-related rotor destruction CANNOT be cured even by "proper" machining. For example, if a rotor develops a "high" spot from heating, and one machines that spot down (even if one does so properly), the rotor will appear smooth, for a time, and will operate vibration-free, temporarily; but the area that required material removal will now be thinner than the rest of the rotor, and heat generated by use will cause a higher temperature rise in that thin part, which will re-warp more readily. Thus the more one machines the rotor, poorly or otherwise, the worse the vibration problem gets; the vibes will return REGARDLESS of how correctly the parts are machined, even if they are vibration-free immediately after machining. The only solution is to replace the damaged rotor with one which has not been subjected to heat-related damage.

Yes we're aware that the floating caliper design and the multiple-piston design are attempts to minimize the effects of unavoidable lateral runout. However, heat damage to the rotor can take other forms besides pure lateral runout, including variations in thickness, and variations in coefficient of friction (such as from heat checking on the rotor surface). There we go again, heat as opposed to poor machining, as either the root or the proximate cause of brake vibrations.

Out of ALL the factors affecting brake operation, the EASIEST one to control and get right, is the machine work. Therefore it is the least common cause of brake vibrations. Factors not under direct control of the mechanic (metallurgy, the driver, the environment) are responsible for the majority of such failures.

That's how it is here on Earth, anyway. Maybe it's different in other universes, I can't comment on that.

I'm really surprised you would attempt to argue this. It's so blatantly obvious, and so common in real-world experience, that I would expect ANYONE who has ever maintained a car over a period of more than a couple of years, to have bumped up against these facts and learned how these things work. I almost feel like I'm proving to somebody that the Earth is round instead of flat.

You torque your lugs right?

If you got the kind of rotors that just slip on, don't have the full hub like some 3rd gens. I read an article I believe in hotrod talking about how you must use a torque wrench and torque your lugs proper as it can cause premature rotor warp / wear from the uneven torque. Some symptom of the slip on rotor. where it wasn't as much of a problem when they used full hub rotors.

Sofa,
I'm not arguing anything and I don't have the time to sit and write a book on the subject, as you have here (very good job, btw). The point I was making is that unless the rotor was manufactured using inferior materials or machining (fairly rare, but they are out there) a "warp" isn't going to appear on it's own or from normal wear and tear at normal driving speeds around town or on the highway.
The example you gave, "pads that sat in one place for a long time and made a particularly rusty spot on the rotor surface" is an example of a pad deposit, not a "warp". Yes, excessive heat can also cause problems when combined with excessive pressure, but where is the heat and pressure coming from? Frozen caliper piston? Corroded caliper hardware binding? Crystallized pads? Valving in the hoses? Plugged combination valve ports? Malfunctioning prop-valve? Bypassing master cylinder? Which is it?
The point is if you feel shaking in the wheel when stopping in a 3rd gen F-body it's probably NOT the rotors being "warped". I'd guess many of us here have ahd a car that shook when the brakes were applied. If the fix was machining the rotors and replacing pads I'll bet the shakes returned fairly soon afterward. The rotors themselves may become damaged from something else, but are very rarely the root CAUSE of vibration.