Hole in '66 master cylinder cover seal??

[Bob S.]

Hi Folks -

Is it correct for the rubber seal under the cover of a '66 (single) master cylinder to have a hole in it?

I just noticed mine has a hole that appears to be there "by design". I have no idea of the origin of my master cylinder, but am shocked the seal would have a hole.....defeats the intent of isolating the brake fluid from moisture in the atmosphere, and negates the need for the folds in the seal to permit movement due to changing fluid lever in the reservoir.

Or is this likely a "screwed-up" aftermarket design?

Regards,
Bob

[Peter L.]

Re: Hole in '66 master cylinder cover seal??

Bob - You got my attention, so I took a look at the seal that is on a NOS '65 -'66 standard brake Bendix master cylinder and I don't see any hole(s) in it. I figure the seal is the original because it is embossed
ABI 20
Bendix
on the center flat section. Pete

[Bob S.]

Re: Hole in '66 master cylinder cover seal??

Thanks Pete!!! That's just what I was hoping to hear, although it means my brake fluid has been exposed to moisture for at least the six years I've owned the car.

I'll replace the seal, purge/bleed the brake fluid and keep my fingers crossed about corrosion from the fluid absorbing moisture. Luckily the car sat in a heated/air-conditioned garage for the last 4 years during a mechanical rebuild, and that I flushed the system with Castrol LMA (low moisture activity) soon after receiving the car....

Regards,
Bob

[Jim C.]

Re: Hole in '66 master cylinder cover seal??

Hi Bob,

I took a look at my notes regarding my master cylinder. I'm fairly confident that it's original to my car. Prior to having the part rebuilt last year, I removed what I believe is the original rubber seal and saved it. The embossing on the interior flat section reads:

ABI 10
Bendix

Also, there were no holes anywhere in the seal.

Jim C.

[Wayne M.]

Re: Hole in '66 master cylinder cover seal??

Here's another (two actually) with the hole. Shown is BENDIX DT 5; my other car has BENDIX DT 16. Good chance they are replacements.

[Paul J.]

Re: Hole in '66 master cylinder cover seal??

Bob,

There aren't any holes in the cap, right? So how can your brake fluid be exposed to outside air? The seal should work on both sides (seal to cylinder, and seal to cap).

Unless I've missed something, the hole in the inside of the seal makes no difference.

Paul

[Peter L.]

Re: Hole in '66 master cylinder cover seal??

Paul - The NOS master cylinder I have has as I indicated no hole in the seal but the cover has a 3/32" dia hole on the side just below the top at 3 o'clock if the cover is oriented so the wording "USE ONLY SAE" is from the 11 o'clock to 2 o'clock positions and reads "normally." Pete

[Paul J.]

Re: Hole in '66 master cylinder cover seal??

Pete, Yep, I thought I might have missed something. All of the pictures that I could find did not show a hole, but it makes sense. I assume that it's there to equalize the pressure as the seal moves. Paul

[Bob S.]

Re: Hole in '66 master cylinder cover seal??

Gentlemen - This is getting interesting. My cover is stamped "USE ONLY SAE" across what I'll call the top, and "70 R3 BRAKE FLUID" across the bottom. Adjacent to the 'L' in ONLY is a hole like Pete described except mine is at a different angular position than Pete's.

My seal is identified:

"BENDIX
DT12"

The seal in my car is installed "upside down" compared to those described above......probably the work of some garage monkey since I don't believe my P.O. did any wrenching himself. Now after stopping to consider the cover and seal designs, it's certainly appears mine is incorrectly installed since the bellows protrudes down into the reservoir without taking advantage of the space provided by the raised dome of the cover.

Anyway, back to the main question: Why the hole in the seal?

Does anyone know when carmakers began sealing master cylinders to reduce the absorbsion of atmospheric moisture by the brake fluid?

Did the '66 basic master cylinder configuration decend from others within the Chevy/GM famiily? ....could the hole somehow be a carryover from older configuration(s)?

Unless I'm missing something significant, a hole in the seal makes me very uncomfortablel!!!

Regards,
Bob

[Jim C.]

Re: Hole in '66 master cylinder cover seal??

I checked one more time, and I also have the same small hole in the metal cap in the same position Pete described, but again no hole anywhere in the rubber seal.

Jim C.