"No feedback brakes" go from nothing to full lock

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dICKj

dICKj

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This problem is totally baffling me. I've been working on cars for over seventy years but have never run across this before. I have a 1974 Duster, 318, power disc brakes, Edelbrock 4bbl (but this problem was there prior to the 4bbl conversion).
My brakes have absolutely no feel in the pedal what-so-ever. The pedal feels like it's not connected to anything. About halfway down, the brakes go to full lock. After trying everything I could think of, I spent a pile of bucks to have a "pro" fix it. New shoes, new pads, new master cylinder, and a new vacuum booster and it's still the same. Does anybody have "first-hand" experience with this kind of problem?
 
When you say the brakes go "full lock" , are we talking about all 4 wheels at the same time? Or just the fronts? Or just the rears?
 
The power brakes on my 74 Duster feel like that, at low speed. The fronts.... Like Nothing, then, at the end they grab. Could be the .484 cam, as it's butt as noted when rolling... but I ain't panic stopped it. New pb, and master. Gonna ditch em and go manual...lots a great Guys and Gals here, and some of the most Experienced minds. This is the place. Please post your results for us? Hope Ya get it fixed!
 
Check the "I'm an idiot" stuff...
  • Is the pedal physically making contact with the pushrod.
  • Does the new MC work right? Lots of new parts store stuff is pretty hit or miss these days.
  • Does it change if you pump it?
  • Are any lines (hard and soft) kinked?
As 72bluNblu mentioned, is it happening on all brakes. One thing that really helped me recently was getting a little IR thermometer and checking brake temperature after a drive. I happened to have bad alignment pulling me left and a bad wheel cylinder pulling me right and they cancelled each other out so the only symptom was rapid brake fade since I was stopping the whole car with just one corner.

Ironracer also brings up a good point - vacuum brakes could be the culprit. The wrong size system and you will be waay overboosted.
 
It could be there is something wrong with the control valve in the vacuum chamber
 
^^^^ this. Depending on the booster brand. You can go from noa to whoa...

Bendix boosters have a rubber disc inside them that controls pedal feel. Leave it out, & the brakes will put you through the windscreen....
 
You could remove the booster and just run manual.

Plenty of stopping power.

Just my 2 cents
 
My sister and brother-in-law had a Buick Lasaber that did this, and it ended up being it had the wrong master cylinder on it.
 
Kinda what I was getting at with the “all 4 wheels or not” thing.

OP says both the master cylinder and the booster were replaced with no change. Obviously it’s not impossible that both boosters are bad, but I would think that’s unlikely. If the master cylinder diameter stayed the same that could definitely be the problem, but, only if the bore diameter is the wrong size.

And if there’s a vacuum issue somewhere else that could be it as well, and wouldn’t have changed with the parts that were replaced.

But without more details from the OP we’re all shooting in the dark.
 
OK, I will first make sure the push rod is making contact when I move the pedal. Then I will make sure that there isn't a vacuum leak anywhere else. If no luck, then I will remove the booster and go manual brakes. What size piston bore do I need in the master cylinder for manual brakes. Anybody got a part number for Dorman, O'Reilly, or Auto Zone?
 
dICKj said:
OK, I will first make sure the push rod is making contact when I move the pedal. Then I will make sure that there isn't a vacuum leak anywhere else. If no luck, then I will remove the booster and go manual brakes. What size piston bore do I need in the master cylinder for manual brakes. Anybody got a part number for Dorman, O'Reilly, or Auto Zone?
Click to expand...
Smaller would give you More Pedal modulation, aka travel, a larger bore kinda squirts it all with a shove of the foot. Alot of us dig the Smaller Bore. Dr Diff has a Sleeved 90s Dakota Master, in Aluminum... that's where I'm going next... sorry, no local P/ns... Rock Auto has ALOT of the same stuff You can get local, so a quick trip to their web can get Ya some #s to cross Reference
 
dICKj said:
OK, I will first make sure the push rod is making contact when I move the pedal. Then I will make sure that there isn't a vacuum leak anywhere else. If no luck, then I will remove the booster and go manual brakes. What size piston bore do I need in the master cylinder for manual brakes. Anybody got a part number for Dorman, O'Reilly, or Auto Zone?
Click to expand...

What size master cylinder do you have now? How are we supposed to recommend a master cylinder without knowing what you already have? Especially if there could be an issue with what you have now?

And, back to my original question, are all 4 wheels locking up at the same time? Because if they are that narrows things down significantly.
 
Here's my take.
Your pushrod is too short.
You step on the pedal and nothing happens, so you push harder and harder until finally you are tickling the M/C. But now you are pushing so hard that when the feedback hits the boosters control valve, BOOM! you got boost!
The Solution is, in my opinion, simple; just increase the pushrod length, little by little until you get the feel that you like. Try an initial increase of three turns, but easy on the pedal on the first test-drive.

Be advised that it is possible to make the pushrod so long, that a panic stop could break the control valve. If that happens, the brakes at the wheels will be activated nearly the instant you step on the pedal, and no amount of pushrod adjustment can compensate for that.
 
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