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1970 Cuda Soft Brakes

70340Cuda

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How much drag should there be on the front disc brakes? Without the wheels on, I’m able to turn them, but there is a decent amount of resistance. If I attempt to spin them and let go, they won’t continue to rotate and will instantly stop. I have a feeling the calipers may need a rebuild, but I don’t want to go through the hassle of it isn’t actually the case.
 

moparleo

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Not knowing the actual history of the "restoration" you may have several areas that need attention. Like the car is supposed to have 10" rear brakes.
Drums and rotors need to be measured. You can't go by the "looks good" . An oversized drum acts like a cylinder that is bored .040 and the pistons are .030. You can see the problem.
All the brake specs are in the service manual and you should go back and verify everything in the system.
Rebuilt calipers are very reasonable.
Diagnostic procedure is to start at the beginning and verify all parts are within specs and working properly.
No assumptions. Verify.
 

70340Cuda

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Not knowing the actual history of the "restoration" you may have several areas that need attention. Like the car is supposed to have 10" rear brakes.
Drums and rotors need to be measured. You can't go by the "looks good" . An oversized drum acts like a cylinder that is bored .040 and the pistons are .030. You can see the problem.
All the brake specs are in the service manual and you should go back and verify everything in the system.

Rebuilt calipers are very reasonable.
Diagnostic procedure is to start at the beginning and verify all parts are within specs and working properly.
No assumptions. Verify.
I greatly appreciate the advice and I will work on verifying every single component on the brakes on the car. It will take me some time, but I’m sure the issues will become clear as I check everything. I will update this thread once I’ve looked over everything.
 

MOPAR Andy

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Just read all of the posts in regard to your breaking issues. I have been chasing this same problem on my 71 383 cuda for the past three years. Three different masters, two boosters. everything else replaced and have bled the brakes every way possible and still the same soft low pedal. I have 4 other MOPARS from this era with the same breaking systems and they all work fine. I was told that maybe it would help if I put in a master with a larger bore but they are hard to find and I don't believe the slight increase would make a difference. I believe the original boosters came with an adjustable rod where it connects with the master. it is a temperamental adjustment and I don't really want to cut and weld the rod for the adjustment. I thought it might be a vacume issue but the vacuum seems to be within spec. Like you I am frustrated and all of the MOPAR experts in my area don't have a solution. Keep me posted and I will let you know if I come up with any solutions
 

Challenger RTA

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I agree with others. It's air in the line. Had the same problem. Mine had new rotors calipers pads shoes parking cable and good drums and rebuilt wheel cylinders. Have everything within spec, drums rotors pads and shoes. adjust parking brake. parts can have wear as long as they are in spec. When you bleed the brakes Did you reset the front metering proportioning vale? sometimes you do nothing some times you simply pull it out and bleed brakes.other time use the clip to hold it or have someone hold it. Try moving both ways . https://www.forebodiesonly.com/forum/attachments/brake-valve-jpg.82222/
 

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70340Cuda

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I agree with others. It's air in the line. Had the same problem. Mine had new rotors calipers pads shoes parking cable and good drums and rebuilt wheel cylinders. Have everything within spec, drums rotors pads and shoes. adjust parking brake. parts can have wear as long as they are in spec. When you bleed the brakes Did you reset the front metering proportioning vale? sometimes you do nothing some times you simply pull it out and bleed brakes.other time use the clip to hold it or have someone hold it. Try moving both ways . https://www.forebodiesonly.com/forum/attachments/brake-valve-jpg.82222/
Thank you very much for the input! I did not reset the front metering proportioning valve. To be honest, I’m not fully sure how to do it, but I will look into how to get that done.
 

Challenger RTA

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Thank you very much for the input! I did not reset the front metering proportioning valve. To be honest, I’m not fully sure how to do it, but I will look into how to get that done.
In the picture I posted it's the silver pin on the bottom of the valve. I moves in and out. if not it's stuck. A pair of pliers my free it.pull it out.if it won't stay when pumping ,have someone hold it or a pair of locking pliers. there is a clip for this procedure.The link was from another post for a different problem.
 

70340Cuda

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In the picture I posted it's the silver pin on the bottom of the valve. I moves in and out. if not it's stuck. A pair of pliers my free it.pull it out.if it won't stay when pumping ,have someone hold it or a pair of locking pliers. there is a clip for this procedure.The link was from another post for a different problem.
I came across another thread in a different forum and a guy was questioning if he had a bad master cylinder or air. A suggestion was to test if it was a hydraulic or mechanical issue. When I adjusted the rear shoes all the way out to where the wheel rear wheels would not spin, the pedal felt quite a bit better. To continue testing I went ahead and removed the front calipers and clamped the piston in its retracted position on both the passenger and driver side. When I went to push on the pedal with the car on, my foot went to the floor with almost no resistance. Would this be air, a bad master cylinder, or stupidity on my part?
 

bc3j

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I have Miller tool c-4121 to hold the metering valve open when bleeding the fronts. I have had excellent success using a motive power bleeder for the brakes. Just remember to remove whatever is used to hold the metering valve open before pushing on the brakes.
 

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Challenger RTA

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Keep it simple. every thing in spec? A bad master cylinder and air in the line,will respond the same. try gravity or force bleed to get fluid in line. I have done my own brakes and others for decades. only one time I had to reset the front metering valve. after thought rubber line collapses and wont bleed? force bleed tool advanceautoparts.com/p/cardone-master-cylinder-bleeding-tool-10-5000mcb/20352246-P?searchTerm=force bleed tool

Master Cylinder Bleeding Tool.jpg
 

70340Cuda

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I went to bleed the master cylinder and noticed that as I pushed the plunger on the back end that’s normally moved by the push rod, the fluid in the front reservoir (rear brakes) would have fluid move through it early on when pushing. It took a bit until the rear reservoir (front brakes) started to have fluid move through the line. At that point, it became pretty difficult to move the plunger as well. Is this normal? Or should fluid move through both lines immediately when the plunger is being pushed in?
 

70chall440

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I am not sure if the piston pushes fluid through both circuits simultaneously or not, I know its pretty close though. If you take a MC apart and look at you will see how it works.
 

Challenger RTA

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Like I said keep it simple a little common sense goes a long ways.remember what know. Think! not being insulting. Disc brakes need more fluid lager reservoir and plunger, it will take longer to fill the plunger. So yes fluid will come out sooner then the other. there is an view of a master cylinder all the same. Take note of the size of plungers. Disc Disc will have larger reservoirs and plungers. If its pushing fluid I wouldnt be to quick to condemn. I think as others said it's air in the line. I know you will resolve the issue angst will drive you. Is this master cylinder original,used .rebuilt,new or other.
 

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70340Cuda

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Like I said keep it simple a little common sense goes a long ways.remember what know. Think! not being insulting. Disc brakes need more fluid lager reservoir and plunger, it will take longer to fill the plunger. So yes fluid will come out sooner then the other. there is an view of a master cylinder all the same. Take note of the size of plungers. Disc Disc will have larger reservoirs and plungers. If its pushing fluid I wouldnt be to quick to condemn. I think as others said it's air in the line. I know you will resolve the issue angst will drive you. Is this master cylinder original,used .rebuilt,new or other.
The angst is definitely driving me. I’m not entirely sure the history on the master cylinder. I have some paperwork from the guy we got it from so I can check back on those as well as look at the numbers on the bottom of the master cylinder.
 

bc3j

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I went to bleed the master cylinder and noticed that as I pushed the plunger on the back end that’s normally moved by the push rod, the fluid in the front reservoir (rear brakes) would have fluid move through it early on when pushing. It took a bit until the rear reservoir (front brakes) started to have fluid move through the line. At that point, it became pretty difficult to move the plunger as well. Is this normal? Or should fluid move through both lines immediately when the plunger is being pushed in?
Sounds right. I plug the ports and push the master cylinder locked level in a vise with the brake pushrod. Once I get the piston to barely move I lightly tap underneath the master and push again. Generally, I’ll get some champagne bubbles that emerge. I’ll tap again and push in until they stop. Repeating as needed. Then the mc piston doesn’t move at all. You know there’s no air left in the master. For a final check, I’ll bolt the master on the car then tap and hand push the pedal while a helper watches for those champagne bubbles. Generally, there isn’t any. The FSM details brake bleeding with the metering valve. Once the master is bled the rest is just bleeding at the four corners in the manner you’re most comfortable with. I just like the Motive pressure bleeding. If the master won’t hydrolock you know the master is bad. If the master hydrolocks you took the master out of the problem and can look elsewhere.
 

Challenger RTA

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So good bad or ugly, you don't know the state of the master cylinder, I agree with bc3j. another method that works. 2 short lines that feedback into the reservoir,pump slow. don't aerate! done it with it on the car too. This has something to do with it or not. Worked a pipe line job,ended up helping the mechanic. Extremely good heavy mechanic eye opening good and Big! he had a difficult time working inside on the co pickups. My job! we both learned from each other. Long story short nobody could fix the F 350 hydraulic clutch. all new parts. Everybody looked at it,the needed a truck to drive.6 other guy had to cram into other trucks.when I got a chance I look at it. heavy equipment had priority . the fix was to pump low and slow and bring up the pedal as it pump up.don't let the pedal come the whole way up until it ready.build pressure. I think it took half hour or better. Note: He was so big he would eat two chicken for Dinner. Every fn day. he wasn't fat ,he was nicer then was big.​

 
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