68 wheel cylinder question
 

I posted a question on a different day about my pass rear brake locking up.

Today it was taken apart and the wheel cylinder was leaking plus it was missing a part for some reason.

So I ordered a new spring kit and wheel cylinders but the wheel cylinders were smaller from oreillys.. Then I returned those and tried another from autozone but still too small so now getting fishy.

Then off to Napa only to get another too small. The guy at Napa looked up the numbers and said his part was what it calls for and even tried matching one for a 66.

So for the heck of it he pulled some big car wheel cylinders and it matched a Bonneville.

Of course I go home and double check my axle and the numbers are what phs says it is.

So my question is could it be possible Pontiac used larger wheel cylinders in a GTO HO car?

If not then I guess someone messed with the backing plates at some point.

The wh says it’s 3.55 which is what phs said it was supposed to be but the larger wheel cylinders stumped me.

Better pic

You may want to inquire how the installer of your new break cylinders determined the correct parts for your car.

I'm not positive but I don't believe there is a difference in rear break cylinders from power to non power, or a GTO HO.

Are both sides the same in the rear?


Frank

Both sides had the same rear wheel cylinders. By part numbers the replacements were smaller and the bolts wouldn’t fit either.

The Napa guy brought out wheel cylinders for a Bonneville and they were the same as what was on mine.. the caps inside were still 7/8 on bonneville and gto wheel cylinders. I think the bolts on mine were 5/16 vs 3/8.

Spring kit and shoes were 68 gto parts...just wheel cylinders were larger.

Wheel cylinder bolts may have been 7/16 actually.

Can you get a number off the backing plate?

Sounds like that's the issue.


Frank

The original wheel cylinders had the dust seals inserted into the housing where most aftermarkets had the dust seals installed over the ends. So the originals looked wider overall. Thats what I thought you were talking about until I read further about the bolt sizes.

Where’s the number located? I’ll see if I can peek under.
It’s really hot here in California. I’m about 35 minutes from the Shaver lake wildfire.

Here are the old ones.

Maybe the backing plates were swapped but not sure anyone would need to.

Should be stamped but I'm not sure of the location, easiest place to look would be the back side.

Is that 35 minutes walking or driving time, get a water hose I'm lookin, lol.


Frank

Lol driving to the town.

PM sent


Frank

Not sure it went through.

Boy, while I can't tell the size of things, nothing is jumping out at me as incorrect.
I have an extra set of 68 backing plates I could dig out and measure exactly if nobody else has a set handy.

Well guess it’s not a big deal having bigger wheel cylinders. I haven’t don’t the front drum brakes yet but if the same size I’m leaning on them being original. The car only has 67k on it.

we are 7/8 bore on our 68 9 a bodys
originals are stamped delco morraine
bolt head sizes do change on aftermarkets and originals ,
front s are a larger bore and left and right specific

My rears were still 7/8 bore but n idea on numbers.

Bolt threads are 5/16 and head size 7/16 on wheel cylinders that were in there.