Brake troubles

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Zachary Davis

Zachary Davis

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Just bought a 1974 duster with drums on the back and disc on the front he put in new booster ( booster is getting vacuum)and new brakes and all new lines but the brakes are hard you can push the pedal down and then it just like hits a wall but the brakes do work you just have to press hard(this is my first classic car ever owned and new to the car scene)

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You got a big cam in that thing?
Not enough vacuum to operate the booster properly?
 
  • Make sure the master cylinder is correct for the application.
  • Also be sure the pushrouds are adjusted correctly.
  • As mentioned above check idle vacuum
  • Check the valve (90deg fitting that goes from manifold vacuum hose to vacuum canaster) is correct and the built in one way valve works correctly.
Mymopar.com has Factory Service Manual you can download free.
 
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Where is the source of vacuum? It should be from the intake manifold directly, not from a port on the carb. Do you have a vacuum gauge to test it? It also looks like the master cylinder may be a drum brake unit. Typical drum brake master cylinders have two equal sized reservoirs, whereas a disc brake unit should have one larger reservoir, also bore diameter is different on disc vs drum if I recall correctly.
 
What does this mean? "hits a wall"
 
Two things, vacuum and maybe too small of bore on the master.
 
Describe the idle. Does it run smooth like a stock engine or does it have a lope like a hot rod engine? "I don't believe there is a bigger cam in it" is very vague at best. You say the booster is getting vacuum? How was that verified? You should remove the check valve from the booster, with the vacuum hose attached either while the engine is running, or remove it and start the engine. Put your thumb over the vacuum check valve and see if it pulls vacuum. Don't just remove the hose from the check valve. That's not diagnosing the check valve. If the check valve has failed so the booster does not get vacuum, there's your problem. Lastly, get a vacuum pump and connect it to the check valve while it's ON the booster and see if it holds vacuum. If it leaks down, or doesn't even pump up, you have a bad booster. It sounds to me like the booster is not working for some reason. I would be concentrating on it first. Once you confirm it or eliminate it as the problem, then move on to the master cylinder. Diagnosing one thing at a time will make it much easier than shotgun guessing. Good luck.
 
missing linc said:
Where is the source of vacuum? It should be from the intake manifold directly, not from a port on the carb. Do you have a vacuum gauge to test it? It also looks like the master cylinder may be a drum brake unit. Typical drum brake master cylinders have two equal sized reservoirs, whereas a disc brake unit should have one larger reservoir, also bore diameter is different on disc vs drum if I recall correctly.
Click to expand...

this guy knows what he is talking about....everything he said is true.....you have the wrong master cylinder on there if you have disc fronts and drum rears.....and vacuum line should be off intake manifold....not carb....and also should be at least be 10 or higher in order for a power booster to work....so check it with a vacuum gauge
 
Your MC is for drum-drum cars, but "should work". Does the pedal feel like it is bottoming out? Unbolt the MC, but not the tubing and pull it out enough to see the rod from the booster. There is usually an adjustable tip. Adjust so it just barely touches the MC piston as you bolt-up. Might have too much gap. If any air in the system, the pedal will be too springy. Need to flush thru new fluid every few years in VA anyway, which will also bleed out air bubbles. If you use DOT 5 silicone, no need to flush.

I prefer the newer 2-bolt alum MC's. You can buy a 2-4 bolt adapter plate ~$30 ebay.
 
Just rev it up a bit and you'll have plenty of vacuum.
If it still acts the same, you probably need to adjust the pushrod a lil shorter, or maybe longer
I start with shorter cuz if you make it too long you could break the control-valve. For shorter, just loosemn the nuts a few turns and slip some shims in between the booster and the m/c. If no change or worse then remove the shims,lengthen the pushrod one or two turns, and try again.

FWI: on a factory set-up, the frontmost reservoir is plumbed to the REAR brakes, through the Combination Valve; which has a Proportioning valve in it, and a safety switch which gets wired to the (!) lite or the (brake) lite to indicate a brake failure. If you have excessive pedal travel, like almost to the floor, this indicates a failure in one end, or the other; you have to figure out which it is.
 
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So I tried a new booster and master cylinder and the same thing happened so I just decided to get rid of the booster and make it manual. They worked when I switched it to manual guess it was a vacuum issue. thanks for the help and the replies did not expect this many
 
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