Rear brake MC goes all the way down and comes back up without putting much pressure in the system. The brake light comes on for just a second. I can feel the piston in the caliper moving just a little. Brand new forward controls. Have bled multiple times, I'm confident there is no air in the system. Anyone have this issue before?

 

don't know if you have a rubber hose to the caliper or not - but we have seen the hose collapse internally and almost stop the fluid from traveling

 

1. I had almost the same issue with my front master cylinder minus the brake light bit. I had installed a new m/c kit that I got from J&P cycles and fought with it for hours trying to get any kind of pressure to bleed my brakes. Had the m/c apart a couple of times. Finally I noticed that the dimple on the back side of the piston where the push rod from the brake lever goes wasn't as deep as the original. If I remember right, about . 030 shallower. So the piston wasn't going back far enough. I cleaned up and used the original piston with new seals and it worked fine.

 

Yes, it is rubber from the caliper to the T-block connector where the brake light switch is.

 

Is there a way to check this hose? Tried squeezing it with my fingers and it wont give, seems pretty stiff.

What would you recommend for a quality replacement?

 

its fluid flow - how good does it blead out

 

simply disconnect rubber hose from caliper (be prepared for a mess, towels or catcher for fluid.) And press on the brake pedal. If it comes out your master and hose is good. And would reccomend piston on the caliper.
if nothing comes out, disconnect hose from t line also, press to see if fluid comes out. (Make sure you keep fluid full, * also use what type brake fluid spec it calls for) -if it comes out the t line, your hose is bad. If nothing comes out, I'd assume it be your master. Also check all fittings.

 

To bleed, it's easy with 2 ppl. One person attach a see through hose on the end of the bleeder fitting on the caliper (recommended, not needed) have other person apply pressure to pedal constant( like trying to push all the way down and hold it there till bleeder screw is opened then SHUT) other person crack open bleeder screw till fluid comes out. Then close. May have to pull pedal up after(slowly) then pump pedal to accumulate pressure and repeat process above making sure fluid is topped off.

 

I went though this.
very similar problem. Rebuilt og mc. Tried a nice, older zel MC, with a good bore. Rebuild it as well. Same thing. And a shitty aftermarket MC. Mixed and matched piston etc.
my pushrod was too short. Wasn't allowing long enough stroke om the piston.
longer pushrod...everything was good.
Maybe try putting something between the pushrod tip and piston, and see if that gets the pedal back.

 

Endless bubbles, even after going thru a LOT of DOT 5 (recommended by H-D).

NOTE: when i first reassembled the rear brake system and bled it the first time, it worked great for about 3 steps on the brake pedal. After that it never worked correctly since.