Large bolt pattern brake HELP! HELP! HELP!

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Tawny Demon

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1971 Dodge Demon manual 4 wheel drum converted to disc/11" drum. retained original distribution block and using disc brakes off a 73 Dodge Dart with large bolt pattern with calipers flipped and bleeders at highst point on caliper. reman "Advance Auto parts" master cylinder that was bled as directions for quick pickup indicated. has firm feel on bench but in car with line hooked up is soft. can hear rear drums engage/disengage as brakes pumped but pedal goes to floor on first pump. will gain some pressure if continue to pump brakes. any ideas? I didnt't use a proportioning valve...is that it? any other solutions? this is my second master cylinder
 
If you can hear the rear brake shoes traveling they may be traveling too far, as in out of adjustment. When adjusted properly you should hear the shoe just barely rub the drums brake surface. Then the shoe will make brakes without making a noise.
Pictures of the brake assemblies are always a great help here.
 
Have you bled the entire system? Bench bleeding the new m/c is good, but you usually have to rebleed the entire system once the new m/c is in.

Sounds like you still have some air in the system if the pedal "pumps up". You shouldn't need a prop valve to get a stiff pedal, although you may need one to run the 11" drums in the back with the 10.87" brakes up front to get the front/rear braking bias right. And make sure your drums are adjusted properly as well like Redfish pointed out, if they aren't adjusted then you may be losing some pedal travel pushing the shoes out. 11" drums also have two different sizes for wheel cylinders, believe they're 15/16" or 7/8". You may need to change sizes to get things balanced out better.
 
I have bled the entire system and do have the correct wheel cylinders. I honestly don't know where to start diagnosing the problem. is there anywhere you would reccomend starting? I don't know if it's a lousy master cylinder or what? it sounded like it was supposed to be an easy swap.
 
Did you catch Redfish's response about making sure the rear brakes are adjusted up?
 
Tawny Demon said:
I have bled the entire system and do have the correct wheel cylinders. I honestly don't know where to start diagnosing the problem. is there anywhere you would reccomend starting? I don't know if it's a lousy master cylinder or what? it sounded like it was supposed to be an easy swap.
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it is a easy swap and yes it could very well be a POS rebuilt master cylinder.
Here's the thing... A master cylinders piston rarely if even travels the length of the bore. The short stoke of the piston wears the bore in only that area.
Reman'd could test good before leaving Coolios hands and fail again during bench bleeding.
I honestly dont understand why people will buy a rebuilt master cylinder, especially one that might be 40 years old and on its 3rd or 4th recycle.
But anyway... good luck with it.
 
You can start diagnosis by seperating out the two systems. Front and rear. If you are sure your brakes are bled properly, disconnect either the front or the rear port on the master and block it off. Pump the pedal a few times and if it gets high and hard, you know to look in the system you unhooked. Or, if there is no improvement, you know to look at the system that remained attatched. If you were to plug off both ports and the pedal didn't improve, it would be a master problem.
 
RedFish said:
it is a easy swap and yes it could very well be a POS rebuilt master cylinder.
Here's the thing... A master cylinders piston rarely if even travels the length of the bore. The short stoke of the piston wears the bore in only that area.
Reman'd could test good before leaving Coolios hands and fail again during bench bleeding.
I honestly dont understand why people will buy a rebuilt master cylinder, especially one that might be 40 years old and on its 3rd or 4th recycle.
But anyway... good luck with it.
Click to expand...

I just had a new master cylinder fail right after being bled and installed. Threw me off my game there for a while.
 
The funny things is that this was my second master cylinder already. I also plan on trying to separate the individuals systems and bleed them tomorrow. And try to bench bleed the master in the car while tapping the master with a wrench to lodge any residual air bubbles.
 
with calipers flipped and bleeders at highst point on caliper.
Click to expand...

Check and double check again. If the calipers are swapped from left to right the bleeder screw will not be at its highest point. You will get a condition exactly as described.
 
You guy's are correct in post's #8,9,12, and that sound's
exactly like where the problem is. You guy's are doing a great job
diagnosing this person's brake problem.
 
What size is the master cylinder?

There are two as I'm sure you know, 15/16" and 1 1/32". Doesn't sound like much, but it can make a big difference. And that's not just a manual/power size issue, each type is available in both sizes. I believe it was a 9" drums vs 10" drums/disks thing. Either way, you can't count on most parts guys to get you the right one.

The calipers being switched side to side shouldn't cause any issues if the spindles are swapped side to side to move the position of the calipers, they bleeder will still be up.

Are you seeing any leaks anywhere? The condition you describe of the pedal hanging up for a second and then going to the floor sounds like a leak. If its not actually leaking fluid out of the system, it could be the master cylinder is losing pressure past the seals.
 
thank you all for all your help. it actually turned out to be my own stupidity: the calipers were on the wrong sides! whoops, I know I'm an idiot, you're all entitled to one free shot. sorry for wasting your time, thank you again for all your help and imput
 
Tawny Demon said:
thank you all for all your help. it actually turned out to be my own stupidity: the calipers were on the wrong sides! whoops, I know I'm an idiot, you're all entitled to one free shot. sorry for wasting your time, thank you again for all your help and imput
Click to expand...

It's never time wasted, Amigo. Good on you for getting her fixed.
 
Tawny Demon said:
thank you all for all your help. it actually turned out to be my own stupidity: the calipers were on the wrong sides! whoops, I know I'm an idiot, you're all entitled to one free shot. sorry for wasting your time, thank you again for all your help and imput
Click to expand...

I didn't tell you that because I thought it sounded good. I fought the same problem for a week. They do look pretty convincing even when on the wrong side though.
 
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