# disc brakes



## rod collins (Jan 12, 2008)

I have installed disc brakes on my 66 gto. My problem is when hard braking the brakes will not lock up. I have changed the power booster, master cylinder, removed distribution block and added proportioning in rear brake line. The condition still exists, brakes will not lock up. Any suggestions?


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## ALKYGTO (Mar 29, 2010)

I'm assuming of course you have adequately bled the system. Did you bench bleed the MC?

Did you use an aftermarket system or is it comprised of all factory parts? What bore diameter on the master cylinder. I'm thinking maybe not enough volume of fluid for the system.


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## rod collins (Jan 12, 2008)

I did bleed the system, bench bleed the MC. I did use after market parts. The MC is a 1 1/8" bore. The car will brake for normal stopping just will not lock up 
in a emergency situation.


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## jmt455 (Mar 26, 2012)

Larger bore=higher effort.
You might want to try a 7/8" or 1" master to see if that gets the efforts where you want them.
Pedal travel will increase, but the effort will decrease.


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## rod collins (Jan 12, 2008)

I had a different MC but it did the same thing. When I try to lock up the brakes I am literally pressing the brake pedal as hard as possible.


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## Hootchi (Apr 7, 2021)

rod collins said:


> I have installed disc brakes on my 66 gto. My problem is when hard braking the brakes will not lock up. I have changed the power booster, master cylinder, removed distribution block and added proportioning in rear brake line. The condition still exists, brakes will not lock up. Any suggestions?





rod collins said:


> I have installed disc brakes on my 66 gto. My problem is when hard braking the brakes will not lock up. I have changed the power booster, master cylinder, removed distribution block and added proportioning in rear brake line. The condition still exists, brakes will not lock up. Any suggestions?


I am having the same trouble. I basically worked with a mechanic and had to measure the pressure in the back and it wasnt nearly high enough.....sucks...Im thinking about going back to drums on the back. We changed the diameter of the lines going to the back to increase the pressure but still not enough to lock it up....ugh. Anyone have any other sure fire solutions to getting this done?


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## PontiacJim (Dec 29, 2012)

Hootchi said:


> I am having the same trouble. I basically worked with a mechanic and had to measure the pressure in the back and it wasnt nearly high enough.....sucks...Im thinking about going back to drums on the back. We changed the diameter of the lines going to the back to increase the pressure but still not enough to lock it up....ugh. Anyone have any other sure fire solutions to getting this done?


Not enough info to help you. Need all the details on the conversion. What you had before the conversion and what the conversion is.


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## Hootchi (Apr 7, 2021)

PontiacJim said:


> Not enough info to help you. Need all the details on the conversion. What you had before the conversion and what the conversion is.


Sorry, the car was stock with front discs and rear drum. I bought the 64-77 rear disc conv kit from Ames part number R605ACB and i bought a new master cylinder, booster and proportioning valve that I was told would help with the system. We eventually measured the pressure and put a smaller diameter line to the back that increased the pressure and the brakes work fine generally but they will NOT lock up no matter how hard you push. I feel like its due to me mixing and matching the old and new. Frankly, im thinking about ripping off everything and just putting a new full system on.


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## PontiacJim (Dec 29, 2012)

Hootchi said:


> Sorry, the car was stock with front discs and rear drum. I bought the 64-77 rear disc conv kit from Ames part number R605ACB and i bought a new master cylinder, booster and proportioning valve that I was told would help with the system. We eventually measured the pressure and put a smaller diameter line to the back that increased the pressure and the brakes work fine generally but they will NOT lock up no matter how hard you push. I feel like its due to me mixing and matching the old and new. Frankly, im thinking about ripping off everything and just putting a new full system on.


I understand the thinking on using the smaller line, 3/16" ?, to increase rear line pressure, but in doing this, you may have cut down on the volume of fluid now going to the rear caliper and that decrease in volume could be an issue.



Do you have enough vacuum to operate power brakes? Was the car a manual brake system or power brakes? Again, did you adjust the rod in the booster? Could be that you are not fully pushing in on the MC piston and not getting full pressure going back to the rear calipers.

If your car was a non-power brake car, did you reinsert the power booster-to-brake pedal in the correct hole in the swing pedal? 

OK, here is what I found on another rear disc conversion set-up that has an E-brake on the caliper, not Ames. However, it could apply to your set-up. I highlighted a few points that may apply or can be checked. Give it a read and you may want your mechanic to try this bleed process. Not it says you will have a "soft" pedal if the calipers are not bled correctly and this kinda sounded like you. 

"5. Check the caliper brackets to make sure they are *parallel with the rotors. Shim or grind the sleeves on the brackets as needed*. Install calipers *with bleeders pointing up.* 6. Install brake hose. Double check all parts for interference and bolts for tightness. 7. Proceed to bleed brake system.

Once everything is installed and before you put the wheels on, bleed the calipers. Air rises so the bleed screw must be at the top. You can start with the normal bleeding methods, but the brakes will not work properly and *you will **have a soft pedal if you do not take the calipers off of the brackets and gravity bleed them to get the last of the air out.*

So read this Pdf completely and note the procedures for the calipers with the E-brakes which your kit should have. You may need to bleed the calipers as noted to get all the air out.



https://static.speedwaymotors.com/pdf/91031919instructions.pdf


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## Hootchi (Apr 7, 2021)

PontiacJim said:


> I understand the thinking on using the smaller line, 3/16" ?, to increase rear line pressure, but in doing this, you may have cut down on the volume of fluid now going to the rear caliper and that decrease in volume could be an issue.
> 
> 
> 
> ...


We tried it with the standard lines first and it was worse. Then we took off the valve and it got a bit better, then we used the smaller diameter and it got a bit better but still now, wont lock up. Its funny because 99% of the time when you just drive around(I use it as a daily driver) its great, and very responsive, but if you hammer on it....thats when it shows its weakness. The car had power breaks already

Thanks for the information, ill go over it with my mechanic and check on everything.


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## PontiacJim (Dec 29, 2012)

Hootchi said:


> We tried it with the standard lines first and it was worse. Then we took off the valve and it got a bit better, then we used the smaller diameter and it got a bit better but still now, wont lock up. Its funny because 99% of the time when you just drive around(I use it as a daily driver) its great, and very responsive, but if you hammer on it....thats when it shows its weakness. The car had power breaks already
> 
> Thanks for the information, ill go over it with my mechanic and check on everything.



Change out the disc pads. Get a set of cheapo disc pads at the local auto parts store. The inexpensive sets are softer in material. The more expensive are harder and typically metallic which don't stop well. The more the metallic, the more effort to get them to stop/lock-up. Their claim to fame is they work best when really hot and don't fade as did the "old" style braking materials. I have used them on late model cars thinking the "premium" pads, or even shoes, are better. They never seemed "better" and what they do do is eat up the rotors/drums up much faster so you have to replace them when you replace the pads/shoes. That is why the have a "lifetime" warranty and will replace the pads/shoes as they make their money on the rotors/drums.

Maybe give that a try. The "soft" inexpensive shoes also work best with drum brakes if you were to have drums - but they are still going to be marginal if doing 120MPH and you have to do a panic stop. LOL


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