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68 Charger Brake gurus.......

Canadian1968

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I have a 68 Charger R/T with factory power Bendix Disc/ Drum setup. I am going to try and chase down a brake problem I have had ever since I put the car back together a few years ago. The first is shake during braking , feels just like rotors are warped. Except its not consistent sometimes it very noticable , other times braking seem totally fine. Seem to be more consistent at higher speed. The other problem is the pedal is inconsistent, sometimes its feel nice and firm, other times feel spongy.

I also get a hissing sound when I apply the brakes sometimes. The sounds is coming from under then dash. I am thinking the Brake booster needs to be replaced . Other thing I noticed is that the master cylinder seems to be leaking out the back bore.

As far as the brake lines go, when I pulled everything apart there was not a "hold-off" valve in line with the rear brakes? Some people say there is supposed to be one, others say their car never had one and as far as I can tell either did this car. I don't think this valve would be associated with any of my problems though?

First thing I am going to do is probably rebuild the master cylinder seems like the easiest thing to do first? ??
 
I think you have answered most of your own questions. Pedal shudder during brake application is usually warped discs or out of round drums.
Hissing booster has an air leak, needs fixing/replacing.
You can have your m/c re-sleeved with s/steel or brass. Then fit new seals.
 
Guess I have to see what shape the master cylinder is in ? Or do they typically always need to be re-sleeved ?

What about the hold off valve ?
 
Got a dial indicator? If the rotors are warped and it sounds like they are.. Measuring runout will confirm it...
 
The metering valve, AKA hold off valve, is plumbed into the front disc brakes, not the rear drums. According to my info., your car did not come with one.
 
The metering valve, AKA hold off valve, is plumbed into the front disc brakes, not the rear drums. According to my info., your car did not come with one.
Was there not something inline with the rear brakes ?
 
Was there not something inline with the rear brakes ?
Yes about a foot back along the frame rail after the distribution block. I think it was essentially an orifice to slow down the fluid to the drums.
 
The metering valve was in the front disc system and the proportioning valve was in the rear drum brake system.
I have a 1968 GTX with the factory disc brakes - no metering valve.
 
There is typically a slight hiss of air as you apply the brakes and this can be heard inside the car if it is fairly quiet. The sound is atmospheric pressure being allowed to flow in to the back chamber of the brake booster.
If with the engine running the hiss is really rather loud or you keep hearing it with the brakes lightly applied - likely the booster diaphragm is leaking.
 
68 service manual.
20221017_125515.jpg
 
Then there is this picture a few pages later . This shows the residual pressure valve in the MC. My MC is also back wards to the one in the picture . The smaller resiviour is closest to the brake booster .

Then also reading that if run on the rotors is more than .005 that the rotor shouldn't be turned? Given the thickness of these rotor .005 doest seem like a lot ?
 

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.005 isn't allot, about the thickness of a red cuxt hair. But enough to cause problems..
 
Yes leave as thick as possible and do not go below the minimum. Only machine off material until the rotor runs true again. I wasn't clear about that point.
The FSM will have the minimum stated in the brake spec section.
Just something to think about I have noticed thin rotors are more likely to warp.
I have also seen them with all the material removed from one side. IMO this is dangerous you risk the rotor flying to pieces.
I believe in replacing them if they are getting close to the minimum thickness.
 
Cast iron M/cls are notorious for rusting & then they fail to seal properly, seals get damaged from riding over pitted rust.
Hence a good move to have the m/cl lined with non-rusting metal.
 
Welll I have a dial Guage arriving today ( should got one a long time ago ! )

I will check the run out of the rotors first . It just seems weird how they explain thing is the FSM. Why would the specifically say not to refinish a rotor if it is past .005 . But then the put a spec of minimal rotor thickness that is considerably less?

Oh well , checking the rotors first because if those are out and possibly close to spec then I am opening a whole new can of worms because those factory rotors are not reproduced.
 
Year One sells the rotor for the Bendix calliper equipped cars.

The FSM may be a misprint.
 
Thickness variation is extremely critical on any disc brake system. If you have the rotors properly machined it will take care of both problems.
 
Well pulled the wheel off thr drivers side and checked the rotor.
Thickness is about .845 I say about because it very difficult to get super accurate measurement with the caliper i have without it touching the ridge on the very edge of the rotor.

I measured run out of the outer face and with max of +.007 and - .001 for total of .008.

The book says a min. Of .816 thickness. SO There definitely some material to work with. Guess I'm going to pop them off and see what they can do .

I see year one does sell them.... but I got sick to my stomach before I calculated shipping and tax up here to Ontario !
 
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