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Brake pedal goes soft and to the floor when I back up?

Dananator

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I have a 67 Coronet 500 with a 426 hemi. Bought it as a unfinished project. I completed it and one thing I was advised to do was put disc brakes on the front. I found 1973 Roadrunner and used the 11 3/4" discs on my car. I used the origonal calipers (rebuilt) and rotors and masteer cylinder. I noticed the master cylinder on the 67 was the same number as on the 73 Runner. After installing the brakes I noticed when I backed up the pedal would go to the floor, I pump it once and it would be hard as a rock. I took it to two mechanics and a brake place and had them bleed the system and adjust the rear brakes. No change. I chedked the rears and no actually adjusted them so I adjusted them a little on the tight side the problem dissapeared for a while but once in say 15 trips it will still let the pedal go to the floor. No once can figure this one out. I put a new master cylinder on and I checked the rear pistion diameter on the rear wheel cylinders and they are the same as th Runners 15/16" bores so the componets match the factory setup so what is causing this? I have a Dana 60 rearend with 4:10 gears. If you have any idea what is causing please pm me or reply to this tread! HELP
 
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I don't know if your problem is related but I'll add this. Caliper knockback will cause pedal to hit floor then will be ok after you pump the brakes. Knockback occurs if you hit the back side of the caliper which in turn push's the piston back in a bit which will cause all pressure loss. It usually occurs in situations like a full turn and the back side of the caliper hits the frame.
 
I don't know if your problem is related but I'll add this. Caliper knockback will cause pedal to hit floor then will be ok after you pump the brakes. Knockback occurs if you hit the back side of the caliper which in turn push's the piston back in a bit which will cause all pressure loss. It usually occurs in situations like a full turn and the back side of the caliper hits the frame.
@Dananator

Good thinking.

For the OP, does your car have a factory sway bar? And if so, did you mount the calipers towards the front of the car or the rear of the car when you did the disc conversion?

If it has factory sway bar and you front mounted them it is possible that the calipers are hitting the sway bar mount and doing what mikespolara says. When you notice the problem is it all the time in reverse or maybe just after departing your garage where you turn the wheel full deflection in one direction?
 
@Dananator

Good thinking.

For the OP, does your car have a factory sway bar? And if so, did you mount the calipers towards the front of the car or the rear of the car when you did the disc conversion?

If it has factory sway bar and you front mounted them it is possible that the calipers are hitting the sway bar mount and doing what mikespolara says. When you notice the problem is it all the time in reverse or maybe just after departing your garage where you turn the wheel full deflection in one direction?
I will check it again but
@Dananator

Good thinking.

For the OP, does your car have a factory sway bar? And if so, did you mount the calipers towards the front of the car or the rear of the car when you did the disc conversion?

If it has factory sway bar and you front mounted them it is possible that the calipers are hitting the sway bar mount and doing what mikespolara says. When you notice the problem is it all the time in reverse or maybe just after departing your garage where you turn the wheel full deflection in one direction?
i had 2 brake shops check it, I will look again, thanks for the tip.
 
I did have it checked twice, one shop verified that I had the right one. One guy Tony suggested getting an adjustable one, which might be the answer. Thanks
 
Are the bleeders on top of caliper? If they aren't could have an air pocket. Post some pictures.
 
You have the rear reservoir on the master hooked up to front brakes? Defective master?
 
No I checked that first thing, and I have 3 master cylinders now. My original, the one off the 73 Runner and a brand new one. They act the same.
 
It sounds like the problem is in the rear drums.
Are the leading and trailing shoes mixed up?
 
Certainly an odd problem.
Have you tried backing the rear brake adjustment until it has no drag at all?
 
That’s the way the car was when I got it. I checked the brakes and they took 3
360’s on the star adjuster wheel before the brakes even kissed the shoes. So I adjusted them slightly tight which greatly helped the soft pedal when I back up. No I may go over the procedure to set the brakes and the parking brake.
 
Mine does the same when I go forward it is OK always thought it was something to do with the auto adjusters in the rear drum brake
 
That’s two of us. I spoke to a nice guy named Tony and he said he experienced a similar problem on his car. So he installed an adjustable proportioning valve. I am going to get on and try it. So it might work for you.
 
An old trick from my apprentice days is to use a brake line clamp to isolate the rear brakes to work out if the problem was definitely in the front or rear system.
I have never come across this particular problem before.
Can you post an update when you fit the new proportioning valve please.
 
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