Nurse! We got a Bleeder...

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MonkeyMadness

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lately I've noticed I'm having a bit of a brake pressure bleed off and I'm not really sure what to point my finger at and talk dirty too. I have a 73 D100 with manual brakes that I personally have never tampered with. Just recently "within the last 4-5months" noticed while sitting at a light with my foot on the brakes the peddle "slowly" begins to go to the floor. Honestly most of the time I didn't even notice, until lately my e-brake light started coming. lol had to google the issue turns out its also an "idiot" light for braking system. I'm the idiot. So like most people I started looking for a leaks and all four corners are dry and no wet spots or leaks in the line system and all pads for both front disc's and rear drums are new within the last 3yrs. Checked the master cylinder and its completely full, so now I'm questioning the Master cylinder which bring me here to ask the question. In all the cars I've owned over the years I've never had a master cylinder give us the ghost and I'm not sure what to look for. I'm guessing the plunger is worn or torn and its allowing the breaking pressure to bleed off back into the bowls which is why I'm having no leaks but loosing pressure.
Am I on the right track or am I missing something that might not be obvious to me at the moment?
 
Yes, internal leak of master cyl, if you have vac power assist sometimes rear seal can leak into booster or under dash if no power assist but you would see lower level of fluid in master cyl.
 
You didn't mention the wheel cylinders in the back. Sometimes you don't see the leak from outside unless you pull the drums.
You are correct I didn't mention them, but I did check and they are dry and appear to show no leaks

Yes, internal leak of master cyl, if you have vac power assist sometimes rear seal can leak into booster or under dash if no power assist but you would see lower level of fluid in master cyl.
Under the pedal area was the last place I check before checking the level of the master cylinder everything is bone dry which is refreshing, but had me perplexed and why I'm focusing on the master.

My guess is master cylinder.
Me too..
 
If you sit in the (preferably concrete) driveway with pressure on the pedal, and it goes down, and it's not marking it's spot, it's gotta be the master.
 
First, I agree the master cylinder is bypassing due to an internal leak... Second you say the wheel cylinders are dry, and externally I'm sure they are.... Buttttt.... To properly inspect them you roll the bottom lip of the outer dust boot off the retaining groove in the wheel cylinder... Good chance they are leaking but the fluid is contained by the dust boots....
 
The Chrys cars here in Oz used the cast iron US master cyl up to about 1970-71. They then switched to a locally made alum m/c & these lasted much longer. Although alum corrodes, it doesn't seem to be as cast iron units rusting.
 
First, I agree the master cylinder is bypassing due to an internal leak... Second you say the wheel cylinders are dry, and externally I'm sure they are.... Buttttt.... To properly inspect them you roll the bottom lip of the outer dust boot off the retaining groove in the wheel cylinder... Good chance they are leaking but the fluid is contained by the dust boots....
If there was a leak at the wheel cylinders, the MC fluid level would go down. The OP said the fluid level has not changed, so back to the MC being bad.
 
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