brake issues, out of ideas...

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70dart/6

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Hi all, I have a '70 dart that is a daily driver. Brakes are manual 9 inch drums. I replaced all the brake hoses when I got the car and had been driving problem free ever since. Recently the brakes developed an unusual problem, while driving in the city occasionally the brake pedal would fade entirely as though I had lost the bleed. Pumping the brakes would bring the bleed back. New master cylinder installed, same problem. No detectable leaks. While bleeding the system during the m/c replacement, the pedal would stop about 2 inches from the floorboard. I'm thinking proportioning valve, I'd appreciate any ideas or insight.
thanks
 
master cylinder new or reman'd ? Either way its most likely the problem.
 
master cylinder new or reman'd ? Either way its most likely the problem.

I agree. Pedal going to floor with no outside leaks is a master that is bypassing internally.

Even though you replaced it, 's what it sounds like.
 
Thanks for the input guys, I'd never say never with the master cylinder but I see no signs of leaks anywhere. I'm wondering if pressure bleeding the system will help, it does bleed like ****, you just never get much pedal. But I will check the m/c again. thanks
 
one chk you can do is pump the brakes a couple of times and then hold... if it stays then its more then likey not the master... but if it starts to go down and bleed off there is your problem...

all the factory prop block does is keep you from loosing your brakes entirely if you loose a hose in either the front or the rear...
 
I tried your suggestion moparkid, after pumping the brakes and holding them the prssure doesn't bleed off. I don't think the mc is the problem. But I'm not sure what else is left, wheels cylinders don't leak, brakes are adjusted. Lines are good, hoses are new. So its either the pvalve or air trapped in the line, or the 2 m/c's I've bought were junk (which is possible, autozone right! haha) I'll keep trying thx

edit: I didn't bench bleed the m/c is it necessary? if so how do you do it?
 
I tried your suggestion moparkid, after pumping the brakes and holding them the prssure doesn't bleed off. I don't think the mc is the problem. But I'm not sure what else is left, wheels cylinders don't leak, brakes are adjusted. Lines are good, hoses are new. So its either the pvalve or air trapped in the line, or the 2 m/c's I've bought were junk (which is possible, autozone right! haha) I'll keep trying thx

edit: I didn't bench bleed the m/c is it necessary? if so how do you do it?


Yes….. Bench Bleeding is required and most likely your problem. It is very ease. You can get a very inexpensive kit with instructions ant any autoparts store.
 
........U say u dont have much pedal........then u need to adj the brakes up.........did u tighten them up till u couldnt turn the drum, then back them off 6 notches....they have to have some drag or there too loose.....kim......
 
I will try bench bleeding and let you know. Fingers crossed!

One thing I'm concerned with, the master cylinder I removed orignally did not have the piston retainer in place, it was a cheesy tab (which bent), so I made a new bracket, but is the piston supposed to be flush or inset in the back of the mc? Mine is flush, I don't see a way air could get in from behind but I'm not sure my bracket design was correct.
 
one chk you can do is pump the brakes a couple of times and then hold... if it stays then its more then likey not the master...

this is not always true, sometimes just the opposite happens and light pressure causes the brake pedal to fall, either way sounds like a bad M/C
 
one other thing, your prop valve is not one at all on a drum brake car-- it is a distribution block, so unless your leaking fluid at one of the connections -- that would not be the problem.
it is either the m/c or 1 of the w/c bleeding off pressure.
Lawrence
 
yup sounds like your cups inside the M/C are warped. bench bleeding is the best way to get the air out. it's a very easy process. how long ago did you get the M/C? maybe you can return it
 
one other thing, your prop valve is not one at all on a drum brake car-- it is a distribution block, so unless your leaking fluid at one of the connections -- that would not be the problem.
it is either the m/c or 1 of the w/c bleeding off pressure.
Lawrence

HOLD THE PHONE AND READ THIS AGAIN, it could be the problem!!!!!


The "block" on those cars may not be a proportioning valve, BUT IT HAS A PISTON OPERATED SWITCH to trip the brake warning light. THAT PISTON COULD BE LEAKING INTERNALLY
 
So this problem just keeps getting weirder. I bench bled the m/c, which worked. I put it back on the car and leaned on the pedal with good results. As soon as the car moved the pedal dropped a bit and a quick drive around the block showed no signs of improvement. The pedal will still disappear on me now and again. Pumping it a few times brings it back. What gets me is why the pedal changes when the car is moving, there is no vaccum booster and or vacuum equipment on the system.
 
If the pedal changes "a little" when you first move, THAT COULD BE the shoes shifting. But once you are moving, and have made about 1 stop, that nonsense should stop.

There are only two possibilities as I see it:

A leak to the outside, losing fluid. You say this is not the case

Master cylinder bypassing internally. Once again, a new/ rebuild cylinder DOES NOT mean a "good" cylinder

Last possibility I posted above is the brake warning switch leaking internally BUT AFTER RE THINKING that could not be the problem because there is pressure on both sides

CONCLUSION

IF IF IF you are NOT losing fluid externally, the MASTER is bypassing internally
 
Could having the auto adjusters on the wrong side be causing problems?

(I don't know here….. Someone mentioned this years ago in my Chebby days).
 
It almost sounds like the residual pressure valve isn't installed in the master cylinder you are buying or it isn't working. You might want to check the application of the master cylinder you bought and see whether or not it has a residual pressure valve in it.

The residual pressure valve basically is a check valve and forces the system to keep 10lbs (or so) of pressure on the brakes. If you don't have that pressure on the brakes, then the springs in the drums (attached to the shoes) will squeeze the shoes inward and drive the fluid back in to the master cylinder. The result is that you have to pump the brakes up. You are forcing the fluid back to the wheel cylinder and re-expanding the brake shoes.
 
Do they even put the rpv's in the new master cylinders? Supposedly all of the currently available new wheel cylinders on the market don't require them because of better seal/cup designs or something. Are you still running the original wheel cylinders? New (not rebuilt) ones would probably only be about $30 for the pair.
 
While bleeding the system during the m/c replacement, the pedal would stop about 2 inches from the floorboard. I'm thinking proportioning valve, I'd appreciate any ideas or insight.
thanks

Edit you have manual brakes so this won't apply.

If you have power brakes might want to make sure the rod is inserted correct between the master and power boster. I only say that because I had a similar problem with the pedal not going to the floor when bleeding and I believe that was the problem.
 
I bench bled the m/c and it seems like the problem is solved! Thanks for all the advice!

One final question, I was at a junkyard today and saw a 76 dart with disc's, can I pull those off and put them on my 70 dart?
 
They'll bolt on but you'll need to change the M/C,porportioning valve,ball joints,spindles,calipers,hoses,etc.This will also give you the big bolt pattern for the front wheels which means you'll have to either carry two spares or change the back axles or rearend.You could grab that from the donor car if it's still there as the bolt pattern will then match the front and if it's the 8 1/4 rear end you'll need the driveshaft as well.

Having the big bolt pattern does allow a much better,and cheaper,choice of wheels so it's a nice upgrade.
 
They'll bolt on but you'll need to change the M/C,porportioning valve,ball joints,spindles,calipers,hoses,etc.This will also give you the big bolt pattern for the front wheels which means you'll have to either carry two spares or change the back axles or rearend.You could grab that from the donor car if it's still there as the bolt pattern will then match the front and if it's the 8 1/4 rear end you'll need the driveshaft as well.

Having the big bolt pattern does allow a much better,and cheaper,choice of wheels so it's a nice upgrade.

Cool, good to know maybe I'll get motivated tmrw and go grab those parts.
 
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