# Need help with brake conversion



## Bensjammin66 (May 12, 2009)

So im trying to save a little cash on my front disc conversion for my 66 Lemans. I have a booster and dual master but i believe its just a drum/drum dual master and not for disc/drum. Does anyone know what I am measuring when they say "bore size"? 1970 chevelles had power discs factory but there masters are 1-1/8" bore size. Let me guess, my '66 poncho has an odd 1" bore doesnt it? So im stuck buying a new booster anyway... 

HELP!!


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## Too Many Projects (Nov 15, 2008)

Bensjammin66 said:


> So im trying to save a little cash on my front disc conversion for my 66 Lemans. I have a booster and dual master but i believe its just a drum/drum dual master and not for disc/drum. Does anyone know what I am measuring when they say "bore size"? 1970 chevelles had power discs factory but there masters are 1-1/8" bore size. Let me guess, my '66 poncho has an odd 1" bore doesnt it? So im stuck buying a new booster anyway...
> 
> HELP!!


Bore size is the internal diameter of the piston and cups in the master. There are 2 main differences between a drum and disc master. 1) the forward reservoir is much larger on a disc to supply the larger amount of fluid needed to move the caliper piston. A drum master has same size reservoirs. Drum brakes have a "hold back" valve just inside where the brake line screws into the master. This valve holds a slight pressure (2-4 lbs) in the brake line to keep the wheel cylinder cups inflated against the bore to prevent leakage. Discs don't require this valve and will hold the pads against the rotor causing them to over heat. If you have a drum master, get a correct master for 69-up. They will most likely all have the 1 1/8 bore. It will just move a little more fluid than a 1" and activate the brakes slightly sooner upon application. The booster is the same for either master, but you need a proportioning valve for the conversion too.


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