1965 Brake Booster and Master Cylinder - no O-ring

[Tmiller6]

Steve Martin once commented he believed robots were stealing his mail. I now believe those same robots took my red box full of o-rings.

I now believe the robots did me a favor. I am not going to put an o-ring on my master cylinder’s square groove. My logic follows:

My booster pushrod has this steel backed rubber seal.

It goes into a pocket on my brake booster.

At the bottom of the booster’s stamped mounting surface is a small groove. (6 o’clock in the photo). I believe this is a deliberate “weep” added by the booster supplier.

Why is it there? In the event the rear seal of the master cylinder fails and drips more than a couple CCs, the brake fluid would tend to get sucked into the booster and eventually the intake manifold. With no o-ring on the master cylinder, the fluid will eventually leak out through the weep groove. Further with no o-ring at this joint, there is no possibility of engine vacuum trying to pull fluid out the back of the master cylinder.

My thinking only applies to my ‘65 with this booster. I don’t know if it’s used on multiple years.

Regards,
Tom Miller

[1Bad55Chevy]

I would run the o ring to prevent a vacuum leak. If you believe it will be fine without it install it and spray the base of the MC with starting fluid and listen for RPM changes.

On a side note if a MC fails it will fill the booster with brake fluid and cause the engine to smoke like it has a blown head-gasket. I have seen this a few times on 2007 and up silverados.

[puddlepirate]

I removed the master cylinder from out 69 Mark last night to clean it up and bend some new brake tubing. Ours looks to be the same design with a less pronounced weep channel (no idea if it's original or not). Ours did not have an oring on either the shaft or the exterior rear of the master cylinder. I would assume the weep channel is there to allow you to see if there is a small leak. I can't imagine the vacuum on the front side of that shaft seal is high, so I'd assume any residual should slowly weep down the channel and not into the vacuum line itself. Just a theory?

[Lee]

I think those robots were actually stealing luggage, but no matter. I think all the Bendix dual diaphragm boosters were designed that way.

When I was running a single MC on the '62, I found myself adding a lot of brake fluid, but the brakes worked fine, and I couldn't find a leak anywhere. Eventually, I found it. Yep, the single diaphragm boosters will allow vacuum to pull the fluid out the rear.

[1Bad55Chevy]

I think those robots were actually stealing luggage, but no matter. I think all the Bendix dual diaphragm boosters were designed that way.

When I was running a single MC on the '62, I found myself adding a lot of brake fluid, but the brakes worked fine, and I couldn't find a leak anywhere. Eventually, I found it. Yep, the single diaphragm boosters will allow vacuum to pull the fluid out the rear.

Did it blow white smoke? For the life of me I can never understand how the booster can be filled full of fluid and the brakes still work properly. Also I can't figure out why you can't feel that issue through the brake pedal..... its things like this that keep me up at night!

[Tmiller6]

Lee, I believe you’re correct. It was luggage. It’s been a while since I last watched that skit.

[Lee]

No smoke. It was a very slow leak, and went on for maybe 6 months before I became curious enough to pull the MC. There might have been a half pint of fluid in the booster.

Don’t ask me to quote the exact physics law, but I believe (as long as the fluid in question doesn't boil) that the entire vacuum side of the diaphragm will still experience the same “negative pressure” boosting effects even if the vacuum side of the chamber is full of fluid. It might be more sluggish, but in the same way you can pressurize air or liquid, you can also “de-pressurize” both.