Rear brake uncertainty

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thc1776

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Just bought a 68 Barracuda convertible. Originally delivered as a 318 auto. The car has been partially upgraded to a 340 by someone who (it is claimed) is very familiar with this type conversion. The car was being sold as part of a divorce settlement and I don''t have access to the original rebuilder.

Problem: Leak in rear brakes -need a rebuild.

I immediately joined FABO to get access to all this knowledge. My shop is populated with mechanics that were all born just before, or after the car was originally manufactured. They are competent and they like the car - but to them it is kind of exotic, and they have little background to chase things down correctly the first time.

So the car now has 340LA block. The carburetion is now 6-barrell from a 1970 340LAT - the 1970 'cuda Challenger Trans-Am sedan version. The trans has been upgraded to a 727, now disk brakes on the front and a 8 3/4 rear end housing. Wheel bolt pattern is 5 on 4 1/2 - I'm told the axles and bearings are new. I'm upgrading to a new M/C and power brake booster. (I'm 77 years old - running out of manual brake leg strength. This car has been on my bucket list since 1971 when I owned a '69 fastback - but, you know, family and kids.......)

The brake shoes on the rear are 1 3/4. The drum (not the original) is 3 inches deep - that would imply 2 1/2 shoes. The drum fits the backing plate.

Two questions: Could I upgrade the shoes to 2 1/2 using the same hardware and backing plate? Is it possible that the backing plate is actually one for 2 1/2 brakes and the 1 3/4 shoes happen to be just centered on the drum? (I'm confident about the hardware, but not the backing plate).

Also, did the disk brake cars come with a brake distribution/balance valve, and if so - where was it located?

Don't mind my being wordy - I've found the most difficult part of problem resolution is getting enough information to be able to make the correct decision.

Thanks in advance...

Tom
 
Not sure why younger mechanics would be confused by drum brakes. Most cars still use them today in the rear (same basic design).

Read-up here on the 8.75" rears. Many used a bell-shaped drum, so the overall depth might have been 3", but less width where the shoes contacted. Those drums are now rare, so someone may have installed a regular drum, which should still work but the shoes would look too thin. You could install wider shoes, but then you might get too much braking in the rear. You don't want to lock-up the rears and spin out.

All disk/drum cars require a proportioning valve to decrease pressure to the rear drums. In 1967, that was a separate device. By 1969, they incorporated it into the "distribution block", which was then termed "combination valve" (plus a front metering valve at some point). I don't know about 1968, but the answer is surely here via search. I have better luck searching from google w/ "forabodiesonly".
 
Pics of what you have currently installed will go a long way to identifying what you need, and what you have!!
 
"Read-up here on the 8.75" rears. Many used a bell-shaped drum, so the overall depth might have been 3", but less width where the shoes contacted."

The machined friction surface is 3 inches. The drum seems to fit the axle and backing plates correctly. That's what leads me to believe perhaps actually these are 2 1/2 inch brakes.

Tom
 
The backing plates are also matched to the shoes, and are specific to a shoe size.
The backing plate locates the wheel cylinders and hence the pushrod hardware.The pushrod centerline distance to the brakeshoe guideways determine 1/2 of the shoe backing plate width. Since I have never seen brake shoes with offset center-ribs, this then implys that the brake shoe would be 2 x the 1/2 width.
Now as the brake shoe width increases the backing plate has to move inboard, so that the matching drum does not rub on it.
So getting back to your question; If the drum is not rubbing on the backing plate, and has room for more shoe, then it likely has the wrong shoes installed. The proof is in the examination of the pushrod angle.If the shoes are too skinny, the pushrods will be running at an angle relative to the backing plate inboard wall or in fact to the car centerline.And that would be bad, as it would tend to rotate the shoes and wear off one edge faster than the other.
 
What might clear up the confusion, Does the car have the small 5 lug on 4" bolt circle or is it 5 on 4 1/2"? The original small bolt pattern 8 3/4 used a specific axle spacing and will only work with the 10" x 1 3/4".
 
When the car was converted to disc brakes, the rear axle was changed to 8 3/4 with the big bolt pattern to match the disc front brakes. So a new axle housing and axles were installed. This means the 2 1/2 brake drums (actually 3 inches across the braking area) were needed to fit the bolt pattern. Where the problem (for me) came in, the converter merely transferred the 1 3/4 backing plate, hardware and shoes onto the new axle housing. When new hardware and shoes was ordered to fit the 2 1/2 brakes, all was OK from an inspection point of view. The hardware kit and wheel cylinder has the same part number for both brake sizes. The difference is that the backing plate for the big bolt pattern brakes has extended stand-offs to move the wheel cylinder outward by 3/8 inch to center the shoes. The wide shoes do not work without the standoffs.

So we went back to the 1 3/4 shoes, not using the outer 3/4 inch of the drum. No big deal - without a brake proportioning valve, the 1 3/4 brakes would lock up anyway under heavy braking. The *** end of a Barracuda convertible is so light you could probably use tennis shoes as a braking medium and get by under normal use.

So a proportioning valve and a full tank of non-ethanol premium to keep the *** end down, and some practice on threshold braking and I'm all set.

Thanks for your attention.

Tom
 
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