Loud Thud Feeling When Braking

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1970Dart198

1970Dart198

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I have a 1970 Dodge Dart Swinger with a slant six, that was converted from Manual Drum brakes all around, to Power Brakes with front discs and rear drums.

Just recently this Thud noise/feeling has been happening when I brake at low speeds. When I drive and come up to stop lights the car will drive fine, and when I brake coming up to a stoplight there is a noticeable THUD when I apply the brakes. There is no difference in power at all after it makes this noise. What could it be? The A904 transmission was not rebuilt, and I put in a new transmission mount and new U-joints with new spider gears in my 7.25'' differential.

Thank you for any ideas or possibilities for this issue, I don't like it and I'd like to fix it before something breaks while I'm driving.
 
Could be many things. One classic test is to put the tranny in N as you approach a stop and then brake. That takes the drivetrain out of the equation. Lift the car by the frame so the wheels are hanging (can do one at a time) and pull on the wheel hard every way, looking for play. My first guess would be worn & thus loose suspension bushings. There is one big one at the lower control arm pivot and two smaller ones where the upper control arm pivots. The adjusters could also be loose.
 
vntned said:
Strut rod bushings maybe...
Click to expand...

that would be my guess ....my 72 swinger did the same thing and it turned out to be a strut rod bushing was missing on one side so it only did it when braking and not on take off.

I would look at suspension because mine did the same thing and I have a manual trans so I would doubt it has anything to do with a transmission.
 
Brake pads binding up in caliper ?
Happened on my Dakota.
Advance Auto Chinese pads.

:burnout:
 
When I pump on the brakes coming to a stop, there is no THUD noise. I took off the cover to my master cylinder and saw that the rubber was fully expanded and the fluid was very low, almost to the bottom, could this be just from driving today?

The THUD noise happens noticeably when I first start the car from a cold start and when I stop for errands and then drive off, when I brake at a stop sign there is one, loud distinct BOOM.

I also checked my Automatic Transmission fluid and found it was a little low and there were bubbles on the dipstick. I put the car in park, I checked it on a sort of even surface, and refilled. When I drove off, there was a louder THUD when I was braking.

Holding the brakes down when coming to a stop is when it always happens. Is there any safety issue right now? I'm going to get it checked out but I dont want to drive it too much if it has the possibility of breaking. Thanks.
 
Your kickdown isn't working or misadjusted - Or the attachment rod fell off. Happens to me all the time. You get a nice loud BANG! and a jerk when you slow down to stop in a parking lot, or go to take off. Reattach rod - no more bang.
 
That's what it sounds like! How do I go about adjusting the kickdown linkage on my 1920 carb slant? Thanks.
 
1970Dart198 said:
That's what it sounds like! How do I go about adjusting the kickdown linkage on my 1920 carb slant? Thanks.
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There's a bracket on the base of the exhaust manifold near where it attaches to the exhaust pipe. On that is a lever, with a rod that goes up to your 1920, which acts like a bellcrank. Sometimes that short rod comes off if the little split ring clip is gone, or if it fell off. Check that. If it's there move on to the long bar with a notch cut down the middle, that is your adjustment. There's a bolt going through it, and a long thin spring. Try and adjust it so that when the rod is thrown all the way back, the carburetor is at Wide Open Throttle. That should be a good baseline to adjust from. If the carb is not all the way open while the rod is kicked back, then adjust on the bolt and slide.

Then see if the problem stops, but that's what it sounds like.
 
Thanks a ton! I just downloaded the factory service manual for good reference. I look forward to checking it out tomorrow.
 
Hopefully that's the problem, and then it's solved, otherwise, it's an easy adjustment to pull out of the equation and then you can continue to deduct things till you find the problem.
 
I found the bolt, it's on the rod that goes back to the transmission, which way should I adjust the rod? Thanks.
 
Don't adjust anything until you make sure everything is properly connected from one end of the linkage to the other. If a lock nut got loose look for an indentation on the part it clamped down on and use that as a start to get everything tightened back down.
 
airgrabber said:
Don't adjust anything until you make sure everything is properly connected from one end of the linkage to the other. If a lock nut got loose look for an indentation on the part it clamped down on and use that as a start to get everything tightened back down.
Click to expand...

Exactly. Grab the throttle lever on the carb with your hand and pull it full throttle. Make sure the kickdown pushes towards the back of the car when you do that. You'll be able to grab the kickdown and push it manually and you'll be able to see if the rod goes all the way back or not. When you get to adjusting, you're just trying to make it go all the way back when the carb is WOT. There's a few other factors, but that's the basic target.
 
An Update: I loosened the bolt and pulled the rod out, thus extending it. I extended it roughly 3/8''. When I drove I noticed the car upshifted at a higher speed. Now, when braking, I first noticed a little thud, but not nearly as bad as before. While driving around there were no issues and the car accelerated and braked fine with no bumping issues.

When I went to park the car, shifting from being in Reverse to Drive, and moving into my spot I felt a little thud like before, nothing bad, but it sounded like it came from the rear/underside of my car.

I'm going to wait a few hours let it cool down and drive it again and see/ maybe expand it a little more .
 

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Have you pushed the rod back by hand to see if it goes all the way back the same as when the carb is at WOT? If WOT doesn't push it back all the way, that's what you're trying to make work. You can grab the kickdown by hand and push it back to see where it needs to be. Hopefully the way I'm explaining it makes sense?
circled_zpsaa8ca0c4.png
The part I circled is the rod that goes from the throttle to the actual kickdown bellcrank. It transfers the downward motion to a sideways push motion. When the carb is all the way open, that rod should be down in its furthest down position, and the kickdown should push all the way back at the same time. If you adjust the rod and the carb is still not pushing it back, try to unbend some of the rod I circled and lengthen it. The rod should have a U in it to make some minor adjustments. The rod could also be installed upside down I think. The ends are similarly shaped. Also, make sure when you push the gas pedal all the way to the floor, that the carb opens up all the way too. Your throttle cable could be out of adjustment as well, which the bolt for that you can see in the picture, you loosen it, move the cable up or down, and then tighten again, and see if it is open all the way at petal to the floor. Ideally as well when driving, if you mash it to the floor, the car should downshift.

If you spend a lot of time adjusting it, and the issue is not resolved, and you're sure the rod goes back all the way, then I'd check a seperate system for the problem.
 
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