67442nut

I'm rebuilding the brakes on the rear of a '67 Cutlass.
Actually, I'm doing two cars at the same time. I've converted both to front disc brakes.

As I recall, the bore of the rear wheel cylinders on the four wheel drum brake cars was 7/8" dia.
and the bore of the rear wheel cylinders on the front disc brake cars was slightly smaller at 13/16"" dia.
Aftermarket 13/16" cylinders are 75 bucks each at Advance!

Several years ago, I bought some Wagner wheel cylinders that were
supposed to be correct for the disc brake cars.
I measured the bore on them. They measured about .005" less than 15/16".

I'd like to save some money and use the 15/16" cylinders I already have.
Am I pushing it to use the 15/16" cylinders when I should be using the 13/16"?
If so, can I compensate for the oversized wheel cylinder with an adjustable proportioning valve to the rear wheels?

I know a lot of guys have done a disc brake conversion leaving the 7/8" rear wheel cylinders
and claimed it worked fine.

66luvr

I can't answer your question with techinal expertise and all the pro and con arguements which go along with that. But I can tell you this, I use whichever is cheapest when I replace the wheel cylinders on the cars I have owned, and I have done this for many years. My brakes always work just fine. Usually whatever the Chevys use is the cheapest.

joe_padavano

The short answer is that the larger the wheel cylinder, the harder the brake force for the same pedal force. The problem with using the larger wheel cylinders with the stock proportioning valve is that the larger cylinders will cause the brakes to lock up sooner. People who claim it "worked fine" have never tested the brakes under panic stop conditions.

One option is to use the larger wheel cylinders for better braking force and use an adjustable prop valve to compensate.

67442nut

Thanks Joe.
I had a conversation on this subject with a friend yesterday and told him I thought the same thing you mention-
that those who said the drum brake rear wheel cylinder worked fine
with a disc brake conversion haven't had to make a panic stop yet.
I thought maybe the problem would be that the larger cylinder would require
more fluid volume causing a delay in the rear brake operation.
I've been thinking of ordering an adjustable prop valve just as you suggested.
Thanks again.

66luvr

Long post.
You said you are working on a 67 Cutlass. So I already know when a question is asked there will always be more than one "idea" posted for answers. But what do the parts stores say? Auto Zone has an easy website to look on, so I checked the bore size on some cars, all GM A body, I looked at only 3 different years, since you say Cutlass I used Cutlass (not 442) Chevelle (not SS) Skylark (not GS) and Lemans (not GTO). The results are this;
66 model year rear wheel cylinder, all the same, 15/16. (I checked 66 because that is my model year)
67 model year Chevelle and Skylark 15/16, Lemans and Cutlass 7/8, no mention of front disc brakes being different from 4 wheel drum. Why were they different? I have no idea.
70 model year, same cars, all 7/8, no mention of a front disc brake option being something different.
No mention of the 13/16 you mentioned.

I am not drawing any conclusion, making no claims, not disagreeing with anyone. I am being like Will Rogers claimed "I only know what I read in the newspapers", in my case it is just what I read on the websites. But. I pose a question, what is the difference, why do some use different ones? 66 all same, 70 all same but different than 66, 67 had some differences. But none were listed as the 13/16 you mentioned. And none mentioned front disc, as in none said front disc cars used a different rear wheel cylinder than a 4 wheel drum car used. I looked at the Inlinetube website as they do have replacement parts and claim to be "experts", it listed the rear wheel cylinders but did not offer detailed descriptions, as in bore size, but seemed to agree on the part numbers being the same on a lot and different on some, as if it said Autozone was in agreement with inlinetube.
Just food for thought. I made my decision many years ago, I will do as I have always done, I will use what works and is the cheapest. Another thing on the works/cheapest theme. Rear rubber brake hoses. When you change rear ends you sometimes run into differences there also. Most rears I have worked with use the same line size for the rear housing from the junction block to the wheel cylinders. But the line from the front to the rear is sometimes a different size. Which requires a different rubber line. Some rubber lines use a separate junction block and some use a block made to the rubber line. There are different length lines for different years. It can be confusing and difficult to come up with the right combination of parts when changing the rears, like a 66 Cutlass 8.2 to a 65 Chevelle 12 bolt rear. I did the same thing as above, I spent some time doing online research of the rubber lines, the sizes of the fittings for the metal lines, the junction block made on or separate, all the info. And there was a lot of lengths and combinations to consider as well as prices, very different prices. I finally got one for a 72 Olds which was longer than some but had the block made on and was less than $20. That was less than half price on some of the other lines/combos. I used what worked and was the cheapest, like I always try to do.

joe_padavano

And of course, the auto parts stores website catalogs are NEVER wrong, especially on a 35 year old application...

(hint, look up Olds motor mounts sometime)

The only application guide I would ever trust is the factory parts book, and even that occasionally has an error or a superceded part. Simply hydraulic system design tells you that a larger wheel cylinder will apply more force to the brake shoes for the same line pressure - that's not "opinion", that's physics. More force to the rear shoes for the same pedal pressure means that they will lock up sooner, again, simple physics.

Once again, the factory designed the braking system for all possible load conditions (driver only to full rated passenger and cargo load) and all braking conditions (simple stop on dry pavement to panic stop in the rain). Unless you've tested your car under that full range of conditions, you can't say "it works fine".

copper128

I'm one of the guys who says it "works fine."

I installed a Right Stuff manual disc brake conversion on my '67 442 and left my original rear drums as they were except for a new set of shoes, later-model aluminum drums and adjustment. I don't have an adjustable prop valve, just a repop original-style barrel valve. This setup has easily and repeatedly hauled this 3800+ pound car down from 112-114mph 1/4 mile drag passes. Even at the short Byron Dragway shutdown area, where you have to be hard on the brakes as soon as you cross the stripe or you'll miss the return road turn-off. No lock-ups.
I've had no problems street driving the car, either.

The SSBC power disc conversion on my '67 Delmont came with an adjustable prop valve. I have it set dead center and have never had to fiddle with it in the 6 years its been there - "works fine"

copper128

Just had to add that the factory drum brakes (particularly on the Delmont) were VERY prone to premature lock-up and required periodic adjustment to keep them (relatively) even in braking force side-to-side, too.

I feel much safer driving these cars since the conversion.