The Stovebolt Site Forums The Parking Lot Big Bolts Ideas on brake failure theory

Last night I was moving my 2 ton around the property since it isn't licensed. Unfortunately I was facing downhill on my driveway when the brakes decided to fail. Thank goodness for the emergency. I chucked the front wheel and backed it back up on level ground.

I have been working with the vacuum lines to get my wipers going. The truck didn't have a wiper motor when I bought it. It did have an auxillary electric vacuum motor mounted on the firewall with a 1/4" rubber line going into the cab with a machine screw screwed into the end to block it off. There was a second line going inside the cab and turning back out, and attached to a smaller steel vacuum line. The electric pump wasn't hooked up or working, but it did have the lines on it. I put a tee into the line that comes off the manifold and continues onto the twospeed rearend. I am guessing that the other smaller line goes to the brake vacuum assist, but haven't had time to really explore. I had to send my vacuum motor off for reuild so I attached the vauum line to the small steel line that I goes to the booster, bypassing where the electric pump would have been. Is there a chance that too much vacuum went to the booster and blew a seal or something? I'll try to explore more on the weekend, but any ideas you guys might have would help. Just a reminder that I am a newbie at this. Sorry for the length of the post.

Hy Super55, If you did have a vacuum leak your engine would run poorly if at all. You might have blown a flex line, or a wheel cylinder, or even the master cylinder, time for some investigating, hope that helps.

Even if you have no vacuuum to the booster, you should still have brakes. All the booster does is make it easier to pull the pedal down. If you have brake fail failure it could come from several sources, including not having the shoe adjusted up properly. Check the fliud level first, if its fine then you have either a bad master, a bad slave in the booster, or the shoes arent contacting the drums. If you dont have any fluid in the master then its going out some place, and should be easy to find.

yeah. I figured it might be the obvious failures, but thought it curious that it happened right after I was fooling around with the vacuum line. I'll check it out this weekend and get back.

oh.. and yeah the engine is running like a ford... er I mean poorly.

I'm also starting to explore this area because my '51 4400 seems to have the same setup as yours. Mine isn't even a roller yet, so I'm hoping to learn from other people's experiences.

What happens if you disconnect the line to the wiper motor? Basically return to the original setup.

Also, as I understand it, the reservoir is designed to give you at least one assisted stop even if all vacuum is cut off to it, as if the engine dies. After that the brake pedal just gets a lot harder to push, but the brakes still work.

Does any of this sound reasonable?

The brake vac line should be pretty beefy really, and there should be a check valve between the brake booster and the engine to prevent backflow of pressure into the booster. It MIGHT concievably give you one good pump on the brakes before it eaqualized & you lose pressure.

What I experienced with my old beast when the brakes failed (the booster was inop as well, but they did work when pumped up) was that the brakes were adjusted poorly so that when I let the pedal come all the way back up, the master cylender would release the back pressure off all of the slave cylenders and the shoes would retract far enough that they didn't contact the drums without a few quick pumps.
I've since rebuilt almost all of the system and properly adjusted the clearance on the shoes. Even as it sits there's a big difference, driving it will show a marked improvement I'm sure.

I wasn't as fortunate as you though, mine decided to let me know who was smarter (not me evidently) as I was aproaching a red light at a busy intersection,... :o

Oh yeah, pucker factor 12 that day!

Good thing about being in low range with a drive shaft brake, it was more than enough to stop the truck. Needless to say, I decied it was better to pop for the tow home.