Ok, I have a 93 softail. I bought it a bunch of years ago and have had my problem since I bought it, but now I finally decided to fix it. I have a PM front master cylinder and a JayBrake 4 piston caliper, the problem is that when the guy I got the bike from bought the stuff, I don't think he matched them up correctly. The front brake has always been harder than hell to squeeze, and it's almost completely useless as far as stopping goes. I know they make different size bores for the master cylinder and the caliper. I'm thinking that he used a master sylinder with too small a bore to make the 4 piston caliper work. I've tried locating part numbers on the master cylinder and the caliper so that I could find out the specs on them, but can't locate any marking other than the brand names. If I remove the brake lines and measure the outlet and inlet holes on both, will that be what the bore sizes are? Anythoughts or suggestions are aprreciated.

 

4 piston calipers use the same bore matercylinders as single piston or 6 psiton for that matter. Forn the sound sof it you have exatly the oposite probelm
I expect whatyou have is a master cylinder for a twin disc set up.
There are 2 things that effect how effective a master cyinder/ caliper set up works. One is the mechanical leverage you get from the brake lever pushing on the end of the master cylinder, whuch is all about length of lever and the postion of the fulcrum.
The second is the the ratio of master cylinder stoke and bore matches up to the caliper. To make thing easier years ago a set of standard master cylinder size was adopted. One for single disc and one dual disc set ups. So all calipers no matter how many psitons they haveare made to work with the single disc master cylinder. The real difference between twin disc and single disc master cylinders is how much fluid they flow for the amount of stroke..IE they need to flow twice the fluid for the same amount of lever movment to move the pads the same distance, the pressure applied by pads is the same.

Little lesson in hydraulics. Its all about piston area versus piston stroke.
forpressure that gives
small piston diatoo alarger piston dia = force applied to small pistongives greater force exerted by larger piston,
and for stroke small piston too large piston you get
stroke of small piston produces a smaller stroke at the large piston.
For example....you move the smaller piston say 1/2" and apply 10lbs of pressure. This will then move the larger piston 1/4" but the piston will apply 20lbs of force. Okay these are very basic and rough numbers.Now if you have more pistons conected to the samemaster cylinder you need to increase thesize of the piston as you now need to move double the amount of fluid to move both caliper pistons the same distance.

Obviosuly I could go on about diferental bore claipers, floating versus opposed piston, radial verus axial mounts and also raidial v axial master cylinders. This is when you start to realise that in 105 years harley still dont use cutting edge braking on bikes that really do need them not due to the speed they do but the amount of inertia they need to over come due to the wieght. I can understand the slow evoloution of the engines as a 45 vtwin is the heart and soul of what makes our beloved bikes what they are, but why they insist on using out dated brakes I have no idea.

 

Makes sense to mee, I wouldn't have looked at it that way, thanks for the input.

 

Again the man is 100% correct! I know this to be true. You have a duel disk master cylinder 11/16" piston. You need the smaller one. You might think about checking out the the condition of all when you are there.

 

i installed a 6 piston hawg halter caliper on my poor braking 06 sportster, worked well with stock master + sintered pads! Hardly has poor brakes + suspension with a few exceptions, its good for the aftermarkets