master cylinder shot?

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diymirage

HP@idle > hondaHP@redline
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71 duster, originally a slant 6 car with 9 inch drums all around, now with scarebird discs up from and 10 inch drums in the back
(about the only part left over from the stock system is the master)

the main issue is the pedal is not consistent
sometimes i need to pump it once before i get pressure, sometimes it is fine

what is really interesting is when i am at a light, and i keep the pedal down it slowly sinks, like there is a leak in the system (there is not)

now, when i set the line lock (plumbed in to lock the front wheels) the car stays put, forever

whatever is going on, i know i am not leaking fluid


does this sound like a bad master cylinder?
and how bad would it be to replace it?
(ive heard of people "bench bleeding" a MC, but not sure what that means
 
Failing master cylinders do bypass internally creating exactly those symptoms. I call this "firing warning shots" and consider us all very lucky that they fail in this manner.
Bench bleeding is just that. Short rubber tubes ( typically included with the new unit ) are attached to route exiting fluid back into the reservoir so to work all the air out of the unit before it goes to the vehicle.
 
the main issue is the pedal is not consistent
sometimes i need to pump it once before i get pressure, sometimes it is fine

what is really interesting is when i am at a light, and i keep the pedal down it slowly sinks, like there is a leak in the system (there is not)

Classic signs of a bad MC. Replace it with a new one, not a rebuilt unit.
 
Classic signs of a bad MC. Replace it with a new one, not a rebuilt unit.

gotcha

should i get one for a 71 duster with drums, or should i get a different one, to better fit the brake setup i have now?
 
You need 1 for disc/drum. Also, power or manual brakes can make a difference.
 
it is manual right now


silly question, how does the brake pedal rod attach to the MC?
 
Here's a pic of the rod. It gets pushed in the rear of the MC and is held in with a clip.

P1030343.JPG
 
With manual brakes I would suggest getting a MC with a 15/16" bore for better pedal feel. I recently replaced mine with a new unit. I believe the application was for a 73 A-Body w/ manual brakes.

Here's my thread...It may help you!

Master Cylinder Upgrade
 
One end of the pedal rod has a rubber ring on it that compresses enough to land in a groove in the back of the master cylinder. So... disconnect neg batt cable so brake lamps are not kept on. Remove the bolt attaching the rod to pedal arm ( under the dash ). Then 2 line fittings and 2 nuts ( under the hood ) and its out. Once its at the workbench, You're going to see a tiny metal finger with screw on the backside. This is the only thing holding the spool valve/spring/guts in the master cylinder. So try to separate rod from master cylinder without pulling the master cylinder apart. Or pull it apart and spill remaining fluid all over. Your call.
All this and much more is found in factory service manuals. Many of those are free downloads at MYmopar.com . If you intend to do these services yourself, that manual is your best tool.
As for which master cylinder to buy... disc/drum units have larger reservoirs, holds enough fluid for calipers. Drum/drum does not.
 
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With manual brakes I would suggest getting a MC with a 15/16" bore for better pedal feel. I recently replaced mine with a new unit. I believe the application was for a 73 A-Body w/ manual brakes.

Here's my thread...It may help you!

Master Cylinder Upgrade


thanks

i just read the whole thread, is the raybesto number what got you the oreileys MC you ended up buying?
 
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