Brake "failure" oddity that has me scratching my head.

[69427]

Amazon shows the available overnight.

But, if they are truly 1/8 in, steel tig rod would probably work. 1/8 bolt, etc.

Thanks, I will check Amazon.

We're on a similar wavelength with your tig rod suggestion. While all my tig filler rods are for aluminum welding, I could go pick up some steel rods. And this morning I was looking at a dust covered pack of 308 stainless stick-weld rods that I considered repurposing if possible.

 

[69427]

Well, I took the car out for a test drive just to see if the vacuum line connection rework fixed the problem. It didn't. I had mixed feelings about that. I felt relieved that my change of the hose setup last winter apparently wasn't the cause of the problem. I also felt lousy, because the problem isn't resolved, and the next track day is this coming Friday, so I'm running out of time. So, time to dig further into the vacuum line and the booster. The first thing I checked was the inline filter that's in the vacuum line. There wasn't a filter in the circuit with the stock booster, but my present booster is an '84 Corvette piece (half the weight of the C3 booster), and there's a filter in C4s, so I just duplicated that setup. i pulled the filter off and shook it to see if it made any rattling noise, and a whole bunch of ultrafine black plastic beads/dust came out. More came out as I continued to shake it. I thought, well that explains the dead second pedal application: the restriction in the filter wouldn't allow enough airflow to quickly work the air/vacuum in the booster. I then looked at the vacuum nipple at the booster, and it was full of those same beads. More flow restriction obviously. I cut the filter (plastic housing) apart, but couldn't really see if it was just the trap for the beads, or the source of the beads/dust. I hooked my shop vac to the booster nipple to vacuum out what I could, and also pumped the brake pedal during that to induce some air flow through the circuit.

So, that's where I am. Long term I need to figure out if I need to replace the booster, but for this week I'm hoping I can bandaid things. I'm going to go buy a new filter for the vacuum line, and if the old filter was the culprit due to age (I bought it new at the parts store, but it did look like it had been on the shelf for quite a while) then the system should work okay this Friday. If the problem is internal to the booster, then hopefully the plastic dust production is slow enough to allow a few hours of track time, and any dust will be caught by the filter. I can always pull the filter between sessions and blow into it in reverse to see if anything comes out.

Again, that's where I'm at.

 

[69427]

I just got to thinking. I wonder if one of those clear plastic fuel line filters would work as a vacuum line filter short term, just to see if any crap is being sucked out of the booster.

The pressure vectors are in the opposite direction, but I might buy one this week to try out anyway.

 

[phantomjock]

Best of Luck. Be Safe!
Cheers - Jim

 

[69427]

The rest of the story: I did a bunch of searching on the web, and found out the story behind the vacuum line filter. Apparently GM had a bunch of brake booster warranty issues and failed parts problems in the early and mid seventies. It seems gasoline fumes were gassing off from the intake manifold and getting into the brake booster, damaging the booster diaphragm, causing brake assist problems. GM's "cure" for this was a charcoal infused filter between the manifold and the brake booster. The charcoal was to absorb the gas fumes until the next engine operation when the fumes would be pulled back into the engine. From a few peoples' comments, GM recommended that this charcoal filter be replaced every year. I doubt if many people did that, but I do wonder if the filter would disintegrate with age and vibration, resulting in a filter and hose filled with charcoal dust (see where I'm going here?). My earlier observation that my booster hose was full of plastic powder was most likely charcoal from the disintegrating filter internals. From other info it appears that this filter was used from 1978 to 1984 model years. (I'm using a 1984 design booster, so I had copied the vacuum details.) So, while I still have the '84 design booster on my car, it's not an '84 vintage piece, so I'm wondering/hoping this ten year old unit is built with whatever improved material is in later boosters that are not vulnerable to gas fume damage. I think I'm going to forgo purchasing another charcoal filter, and just see how things go.

If I had heard about this issue thirty years ago it would have surely saved me time and aggravation this past month.

 

[Red77]

It sounds like you may be on to solving the issue. I've never seen one of these booster filters before, just your usual check valve and typical issues were always leaking vacuum lines or a bad check valve. This could very well be the smoking gun as the clogged filter could create a restriction that would effectively extend the amount if time it takes to build the vacuum back up in the booster, just like if you were feeding the booster with the same size line on the vacuum advance can on the distributor. Looking forward to hearing the results at your upcoming track day.

 

[BBShark]

The rest of the story: I did a bunch of searching on the web, and found out the story behind the vacuum line filter. Apparently GM had a bunch of brake booster warranty issues and failed parts problems in the early and mid seventies. It seems gasoline fumes were gassing off from the intake manifold and getting into the brake booster, damaging the booster diaphragm, causing brake assist problems. GM's "cure" for this was a charcoal infused filter between the manifold and the brake booster. The charcoal was to absorb the gas fumes until the next engine operation when the fumes would be pulled back into the engine. From a few peoples' comments, GM recommended that this charcoal filter be replaced every year. I doubt if many people did that, but I do wonder if the filter would disintegrate with age and vibration, resulting in a filter and hose filled with charcoal dust (see where I'm going here?). My earlier observation that my booster hose was full of plastic powder was most likely charcoal from the disintegrating filter internals. From other info it appears that this filter was used from 1978 to 1984 model years. (I'm using a 1984 design booster, so I had copied the vacuum details.) So, while I still have the '84 design booster on my car, it's not an '84 vintage piece, so I'm wondering/hoping this ten year old unit is built with whatever improved material is in later boosters that are not vulnerable to gas fume damage. I think I'm going to forgo purchasing another charcoal filter, and just see how things go.

If I had heard about this issue thirty years ago it would have surely saved me time and aggravation this past month.

So the purpose of the filter was to filter the fumes from the booster that re-entered the engine at shutdown? Wouldn't the vacuum in the booster pull in some small amount of engine air/fumes (when the engine was shut down)?

 

[69427]

So the purpose of the filter was to filter the fumes from the booster that re-entered the engine at shutdown? Wouldn't the vacuum in the booster pull in some small amount of engine air/fumes (when the engine was shut down)?

I'm not entirely clear on what you mean in your first sentence, so I'll just respond to the second sentence.
I think we agree on the second sentence. In a perfect world the checkvalve in the booster wouldn't allow any (ambient pressure) air from the intake manifold to leak into the evacuated booster. I suspect that the checkvalves were imperfect (like most things on a Chevrolet), and the booster vacuum level gradually pulled air and gasoline vapors into the booster interior, damaging the diaphragm. The charcoal in the filter was there to soak up the gas vapor in the hose to prevent or reduce the amount of fuel vapor entering the booster after shutdown.

Given that Chevrolet stopped equipping Corvettes in 1985 with this filter, I'm assuming there was a material change in the diaphragm that no longer required this protection/bandaid.

 

[69427]

Update: I went to a track day yesterday. Recall that I found the previous brake problem was the charcoal filter in the booster vacuum line had disintegrated and filled the vacuum line full of charcoal dust, restricting the manifold vacuum flow from the booster. A few weeks ago I vacuumed as much of the charcoal out of the line and booster checkvalve as I could, and then installed a clear plastic fuel filter in the line to catch any of the remainder before it got into the engine. To my relief the brakes worked perfect all day yesterday. I was happily surprised that the fuel filter wasn't restrictive enough to cause any noticeable loss of assist. Looking at the filter I did see about a half dozen small pieces of debris, so I'll probably keep this filter hooked up for the remainder of the season.