I have a 72 F100 w/360 C6. When I release the brakes from a stop I hear a "clunk" sound? It appears to be coming from either the tranny or the rear end but I am not really sure? When the sound doesn't happen after releasing the brake it will sometimes happen just shortly after accelerating from a stop but at which time it isn't as hard of a "clunk" sound. The U joints are new and I have double checked for any slack in the drive shaft and haven't discovered any. Any ideas what I should look for next?

 

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Rear Brake springs ?
Brake shoes hanging up and release when the drum turns.?
That's my guess.
Dennis

78 F-150 429CJ 465,000 KM.(Original Owner)
68 Mustang 289 Sunlit Gold 80,000 Miles(second Owner)
64 1/2,260 Pre-World's Fair Mustang #8092
64 Fairlane 500 S/C,waiting for a 390 4-Spd

If The Girls Don't Find You Handsome,
They Should At Least Find You Handy!

 

I have the same problem in my '67. I have new u-joints and trans mount, properly function brakes, and have retorqued the rear springs figuring it might be a shift of the axle housing. My last gasp idea is that it is the transmission suddenly downshifting to low upon acceleration. Why don't you try stopping with the trans in low, reaccelerate, see if it still happens.

Maybe a transmission guy could jump in here.

 

Thanks for the replies and the suggestions. I was wondering about the brakes at first too but it sure sounds like a strong clunk and not a hallow clunk like I would expect to hear from brakes.
A couple of times while at a stop sign, anticapting the clunk to happen once I release the brakes, I have put the tranny in neutral then back to drive to see if it would happen and it hasn't yet with that experiment. That is what is making me wonder if it is tranny related or rear end related.
The tranny shifts very smoothly up and down and can barely be felt shifting from 2nd to 3rd. I will try to experiment with it in low and see what effect I get.
I have solicited some opinions from co-workers that think it may be in the rear end? Something about the pinion? I am not real savvy about rear ends. Only the wifes ;-)
Thanks,

 

If you haven't found the problem yet...

I have a similar problem with power brakes in a 78. The booster is worn out(?) and does not provide enough force to return the foot pedal. I hear a clunk that resonates thru the floorpan shortly after releasing the brake or accelerating. Have someone watch the brake pedal while you are driving. On my vehicle it sticks about 3/4 to 1" from the full up position then suddenly releases with a clunk. My truck is due for a massive refurbishing so I have no idea what exactly will fix the problem. I do hear a vacuum hiss from under the dash, and my brake lights stick on sometimes and will run the battery down.

Good luck.

 

I know exactly what your problem is. I had it too. Your tranny is not dropping into 1st gear when you come to a stop. When you hit the gas in "clunks" down into first. Change your filter, make sure your vaccum line going to the tranny is in good shape and consider a mild shift kit. I got mine rebuilt and it went away.

 

4d,
I know this reply is a little late but i may have your solution. If the trans is working properly. Then its one of two things. Are your axles leaking. If your axle bearings are worn bad enough the axle will not be centered with your brakes. This will cause them to bind with the drum and unlock when released.
My favorite is the driveshaft "clunk". If the axle housing is rotating forward when you brake and rotating backwards when you accelerate, this could cause the driveshaft yoke to move forward and backwards excessively, causing driveshaft clunk at the transmission. Check your leafsprings for cracks. Check your fasteners both at the axle and at the body. This would be more able to happen on a one piece shaft rather than the two piece shaft. Let me know what you discover.
Good Luck, Have Fun,
KingFisher

 

Hey KingFisher, the scenario about the axle housing matches a suspiscion I've had about a drive line clunk my one piece drive shaft to my open nine in my 300-6 (C4). I had the tranny rebuilt not long ago and notice at times a driveline vibration at freeway speeds as well - especially if I hit the gas hard to downshift when getting on the freeway. I hope to be doing my rear leaves in a couple of weeks. I'll make it a point to post if it corrects the symptom.

 

 

If you haven't found your problem yet here is a possible cause. The tailshaft bushing on the tranny may be worn and loose. This will cause the slip spline to bind as it tries to go in and out.

Bear Tracks

 

Tracks,
Would this problem also cause a trans fluid leak at the tail seal?
KingFisher

 

I have this same problem in a Jeep Cherokee. In it, the rear leaf springs are sagging a little, and they slip on each other when the brake is released (from a hard stop). The springs are actually wrapping up a bit upon hard braking, and then releasing when the brake pressure is released.

 

I have the same problem in my F250 with a 4spd. I noticed when I lift the rear end up and put the trans in neutral there is a lot of play in my rear end at the pinion yoke. I havent pulled the cover but I think it may be bad pinion bearings.

 

ALong the same lines as this question,my '78 f100 with 302 and auto has a real bad clunk if you let off the gas suddenly and get back on it.Feels like the rearend but when I pulled the 3rd member the ring and pinion had very little play with just a bit more in the spider gears.Any ideas?

 

Guys,
Let's not confuse two separate issues. If you jack your truck up and place it in neutral you should be able to rotate the driveshaft by hand and notice upto a 1/4" rotational play. This is called "lash". This play is caused by the distance between the pinion gear and the "D Ring". There can be a great amount, 1/4 inch, and the ford 9 inch is still fine. When the lash is excessive you will hear a "clunk" from the rear when doing to "drive to reverse" or even a "neutral to drive". This play can be increased when your "pinion end play" is excessive. If you grab the driveshaft and push/pull on it you may see your pinion flange move in and out of the rear end carrier. If it moves more than .012 inches, 12 thousandths, Check the spec before you quote me, it could be much less you need to tighten the diff output flange. This is difficult and should not be over tightened. I can only do it on a bench with a large vise.
Secondly, the "clunk" I'm talking about is from the entire axle housing rotating, causing the diff nose to go down during braking and up under accels. This in turn causes the driveshaft yoke at the trans to move in and out of the trans and literally bangs, or bottoms out, the yoke in the trans. I work for GM truck and Bus in Detroit for a short time and its a problem commonly seen on trucks that have suspension resembling cars, i.e. wimpy. Look for a flimsy rear suspension i.e. F100 and you've got a good canidate. Add a leaf spring and watch the problem go away.
Good Luck, Have Fun,
KingFisher