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Intermittent brake failure

E eugenioinnocenti#3371 #2417
Here is a tricky one for the experts. I was driving along at 40mph and had to slow down. The brake pedal went 3/4 of the way down before starting to brake very weakly. There was absolutely no pressure in the system until reaching the 3/4 point in the travel. I had the impression that either the front or the rear brakes were still working a bit. As I was in a state of shock I couldn‘t do much thinking or further testing. Things returned progressively to normal after actuating the brakes 2 or 3 times.
This has happened a total of 3 times with 3 different drivers within a driving distance of 100 miles. There are no external fluid losses and the system has been recently purged. The brakes have been recently rebuilt with new seals. The car did not pull to one side when I braked.
The brakes are completely stock including the proportioning valve.
I would appreciate some help as I intend to keep being part of this helpful community.
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pantterapantteraMember
Suggested by Mike Drew during my recent Holley/Edelbrock update, as the result of a narrowly avoided "Drewsaster" while he was a passenger in a GTS with those same brake failure symptoms was to replace both (5.5 ft. rear and 18 in. front) vacuum brake booster hoses. They are now both very high on my to do soon list, and MUST be vacuum, NOT coolant hose.
Your vacuum may vary........

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S simonMember
quote:
Originally posted by PanTTera:
Suggested by Mike Drew during my recent Holley/Edelbrock update, as the result of a narrowly avoided "Drewsaster" while he was a passenger in a GTS with those same brake failure symptoms was to replace both (5.5 ft. rear and 18 in. front) vacuum brake booster hoses. They are now both very high on my to do soon list, and MUST be vacuum, NOT coolant hose.
Your vacuum may vary........



PanTTera , you are wrong, it's not a vacuum isseu.
When you have a vacuum isseu than the pedal press must be harder than normal.
The problem from Dr. is that he push the pedal allmost to the bottum the first time .
and comes up after a few pushes .

The M/C has a bottem press valve that keeps a little pressure into the system , or maybe the seals are leaking from the 2 M/C circuits.
E eugenioinnocenti#3371 #2417
Larry, sorry for the confusion. I was later told that the m/c got new seals and was tested on the bench.
I repeat, I‘m not an expert but if it was a vacuum or boost problem the pedal wouldn‘t go that far down without actuating the brakes. I would have pressure from the beginning but have to push damn hard on the pedal to get any braking action.
If one of the calipers had been blocked, the car would pull to one side or block the wheel.
I believe Simon is on the right track but I am still open for further suggestions or explanations.
I‘ve put a spare american style m/c on this evening. I‘m going to test this for a while. If you don‘t hear from me soon then it wasn‘t the m/c.

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