Hello all,

I had to replace my rear drum brake cylinders, after discovering a leak in one that almost emptied the rear chamber of the master cylinder, which to my knowledge goes to the front disc brakes; however, after getting everything back together I still have weak braking...so, two questions: 1) How did a leak in a rear brake cylinder empty the disc brake fluid chamber (or is that one actually for the drum brakes)? 2) I found a leak in the vacuum check valve, which could have caused weak braking, but when changing the check valve I clumsily removed the grommet and it fell into the vacuum booster - will that affect it's operation? I looked at schematics of the unit on-line, but it's not clear whether or not the rubber grommet fell into a place that may cause problems - any suggestions? I don't really want to go through the a$$ pain of replacing the vacuum booster, although it is 49 years old!

Thanks, Joe

 

I don't believe that broken grommet falling into the booster will hurt anything. I have had MCs just leak if full parked on hill sides because the rubber under the cap didn't seal well because the rebuilt MC had a not so stout cap. I'd just replace the wheel cylinder in back since you know one leaks, bleed, and watch my levels anyway.

 

Thanks, but I did replace both cylinders, and after bleeding they both work properly; however, the effort to stop the truck is excessive, leading me to the vacuum booster, which had a cracked check valve...I replaced that and will test tomorrow. If I still have the same problem, I'll endure changing the vacuum booster, which is the next thing to check I think?

 

You said the grommet fell into the booster. I saw that you did replace the check valve, I'm guessing with a new grommet. I don't know that your booster is good, but you can test it by

• starting the truck without touching the brake
• gently slightly rev it up and then let of to build vacuum
• shut the engine off, still no brake
• with engine off, apply the brake a few times.... if it is holding vacuum (as it should) it should be obvious in the pedal
• keep depressing the pedal a few times, the pedal will get harder as you use the vacuum
• once vacuum is gone, hold foot on brake, restart the engine
• you should feel the vacuum build and the pedal go down as boost comes into play.

 

Not seeing the master I cant tell what chamber is for the disc or drum, but the larger chamber on a disc brake master is for the disc side.

If you dont have any leaks and the rear chamber keeps loosing fluid it is going either in the booster or down the inside of the firewall and being soaked up by the rug.
The fix is to replace the master as the seal between the fluid and the rod has gone bad and yes this happens.
Dave ----

 

In rare cases, I have seen the engine vacuum pull fluid from the rear chamber of the master cylinder and burn it through the engine. It's usually a very small amount that you may not even notice except that the rear MC chamber goes empty over time but you can't find a leak. If all else fails, replace the MC. If the MC has been leaking into the booster a long time, the fluid will eventually ruin the diaphragm so the booster will need to be replaced, too.

Dave J

 

Don't recall if my OEM on my '77 F150 did that, but over many years it sure seemed to grow weaker, and all the paint below the MC was about gone on the Booster front surface. I just knew the truck needed a brake redo, which it got. I noticed that on my then new reman F350 booster there is a seal in front of it that seals the rod to the housing, and under the recess where the MC fit, was a groove to drain any seepage from the MC before it got into the booster. I just looked up a new booster like would have been used OEM, it too has that groove and Booster seal.

Oddly, I am thinking my original OEM maybe didn't, and at the time I was concentrating on the swap and didn't take a pic there. Looking at product photos now, the replacement for that part does however, just like this. I'm wondering if originally in the '77 with original OEM, if the Booster had a pushrod seal and if the MC sealed to the Booster with no groove?


But it works as evidenced in 2024 when I found the F-350 MC had leaked, and as I was replacing it. The crinkled paint makes it look worse, but that seal was only wet at the very bottom, and the push rod was dry so the drain worked, and if the push rod seal didn't seal, the same groove would have prevented a vacuum build up there.


Notice how the pushrod seal has the pushrod held up.

This picture below seems to confirm my recollection, as the push rod is drooping .... but it is extended. Maybe is seal that is black, but just wore slam out and loose?



I had removed the MC first, then the booster, then I had set it up on the already old air cleaner out of my way.
Maybe it had just gotten stuck in a seal when I pulled the MC loose? I would have thought a seal would hold it up like the first of this trio of pics inside A booster 77&79F250 put up.




I now wish I had paid closer attention.

Anyone else know?

 

No leaks around the master cylinder or the vacuum booster; in fact, the only leak I have found was at the left brake cylinder...

The job is done now - I took my booster off and it was clear it had given up the ghost - with cleaned and buffed mounting accessories and a new vacuum booster, all is well!