ok guys, i have a mix/match bike but i need to get some rear brakes working. the rear caliper is a 81 FLT caliper. it seems to be fine, but the master cylinder seems to be a little shot. anyway, since the forward controls are pretty crappy already i'm planning on replacing them.

so question. i've seen forward controls with 3/4 and 5/8 master cylinders. i can't for the life of me find out what the stock caliper would call for as far as bore size. so should i go with the 3/4 or 5/8 bore? please note that i will be using the stock 81 FLT rear caliper AND the stock FLT brake lines!!

 

the length of the pedal and bore size have to be matched to get the proper pressure to the caliper this quote for auto but same theory.
when I set up race cars I would use a pressure gauge plumbed in the the lines at the caliper front and rear to get the correct bias. for a drag race car we want more brake to the rear due to the much larger tire in the rear. motor bike of course we don't need to bother with that since we have control of the brakes on the fly. I would guess if you got forward control matched from the manufacture it would be set up correctly but always worth check the pressure at the rear.
I helped a guy with a car that stopped horrible at the top end of the 1/4 mile. after checking out the pressure we had to add 2 inches length to the brake lever to get the pressure up enough to stop the car. that's all it needed to make the car stop better. hope this helps
BTW most the after market M/C are 3/4

 

5/8" bore master cylinders are single disc, front or rear.
3/4" bore master cylinders are dual disc front.

 

BTW, the stock rubber brake lines don't age well. They lose their rigidity, become elastic and swell under pressure, reducing braking performance (spongy feeling brakes).

At a minimum, replace with new hoses. For superior performance step up to braided steel lines.

 

thanks grinder. i have ordered all new brake lines (both the steel and the rubber lines), the caliper seems fine. the rear brake will slowly set and lock up the rear tire right now, that is why i'm redoing the system.

since i'm using the stock caliper i wanted to know what the stock master cylinder bore was so i can make sure i get a new one that will work.

so now i know to look for a new set of forward controls with a 5/8 bore master cylinder!!

 

I don't believe that applies to the rear..that is why they make both sizes for the rear. The FLT is a really big caliper...you need to check what size your MC is now. The 3/4 and 5/8 MC use different size banjo bolts...so if you buy the wrong one it will not work with the stock brake line.

 

You are correct; the 79-84 FLH and FLT used the 3/4" Kelsey-Hayes master cylinder.

 

the master on the bike now is an aftermarket one and it is not marked anywhere on it what the bore size is. i guess i'll have to tear it apart and mic it.

it also doesn't have a banjo bolt on it, the hard line screws directly into the master as it does into the caliper.

 

 

unless you want to spend the $$ on new controls why not build a new lever putting it above the foot peg with a straight piece to to the peddle and get the leverage correct?
looks like you would need to be standing right on top of that peddle to get it to do anything? just thinking out loud. you might have to get creative to clear the exhaust pipe... I'd guess that is why the lever you got there looks like it does.
not the best image but this is what Deans brake looks like and pretty sure it is a 76 and stock stuff

btw
since I didn't remember I went and measured my M/C bore on the aftermarket cyl. I have and my 91 fl both are 3/4"
I don't think the caliper would care what bore Cyl. you have so long as you get the correct pressure to it to stop the bike.