Back Brake

[jax11]

Hello, Have '79 1100 SF

Rebuilt back master cyl as per man > Problem is that after what I think is a bled brake system, the master cyl piston has no return pressure - like spring is too weak inside . The piston eventually drops back down after like 10 / 15 sec. Did I do something stupid or is their still air ?? Any help appreciated > Jamie

[XSSIVEUS]

Hi Jamie,

Could be several items:
1. Not all air is out. The 1100s have interconnected front and rear brakes. I'd start with this as it is easy to get all the air out if you follow the correct practice of push lever and hold, slightly open bleed valve and let a little air out, close valve, release lever. Repeat the above a bunch of times until no more air, even little tiny bubbles come out with fluid. Make sure you don't run dry in the reservoir as after most air is out, you'll consume lots of fluid.

2. Piston was not polished and free of surface defects. I've had some that were a bit sketchy, but I started with 600 grit, then 1000, then 1500 and finished with 2000 grit paper to clean and polish. If you have corrosion and pits, toss it and order another. Plenty available on line.

3. Piston seal may be twisted. When you install this and other parts, always coat all moving parts with clean brake fluid. At the end of the stroke when applying brakes, the square seal acts like a spring. It has deformed slightly and then pulls the piston back. If everything isn't square, it won't move correctly.

4. Torn piston seal. Normally you'd see a leak if this happened. Seen any?

5. Pad grease not applied to the frame surfaces of the pads or the spring and/or the metal slides that the pads move in. A very light coating is needed to ensure everything moves freely.

6. Wrong parts. Best to buy the right parts and were designed for the caliper.

7. Pad securing screw not installed correctly into the receiver pad.

There are probably a couple of more things that will make your life miserable, but this is a good list to start. Take your time, clean the crap out of everything, use all new parts, coat everything in fluid when assembling, and don't accept "oh that's good enough". It's your arse that's on the line so make it perfect. Hope this helps.

Glen

[DiverRay]

Rebuilt back master cyl as per man > Problem is that after what I think is a bled brake system, the master cyl piston has no return pressure - like spring is too weak inside .

OK, so the BIG question is DID YOU BENCH BLEED THE MASTER BEFORE THE INSTALL?
Before looking anyplace else, try the master.
1. Remove the brake line from the master.
2. with a finger on the hole for the banjo bolt, S-L-O-W-L-Y apply pressure to the master.
3. Allow the air to escape past your finger, but keep it secure over the hole.
4. Let the master retract keeping the hole plugged with your finger.
5. repeat until you have brake fluid ONLY(no spitting sounds) out of the hole.
6. re-connect the rear brake line and then bleed the system at the rear caliper.

[BA80]

Are you sure its the master cylinder sticking and not the foot lever or linkage? I've had several that the rod the pedal mounts to that runs through the frame was really sticky. Have you taken that out and cleaned the rust and gunk off of it and out of the tube it goes into?

Some marine grease to lubricate when you put it back together.

[xs11lover]

If it were me I’d go with something stupid first!

[Bonz]

-Looks like he has a 79SF. The front/rear brakes are in no shape or form connected.

It is possible to have air in the system at the MC that no amount of bleeding will remove, from experience.

May need to bleed it at the master cylinder like DiverRay said, some air just won’t/can’t make it to the caliper especially after a total take-down of the MC.

Double check everything’s doing what it should in the MC. If the MC worked before the rebuild, simply taking it apart and putting it back together again may reveal something and you will find your issue. Like XS11lover said, check “stupid” first. We’ve all done something silly if we have worked on bikes long enough.

Rear master cylinders are the same across the Standard and Special models from 78-81, so if you got a kit for a rear MC it is the correct kit.

[jax11]

Thanks

Hello Alll First Thanks for just being there >>>

So yes 1979 xs1100SF and rear has own master cyl. I did take brake lever stem out and cleaned and lubed, was part of rebuild. I used all new parts to include piston and seal in caliper and in master CYL new seals and piston also. Was a little confused about the DIRECTION of the 2 seals on the piston rod in master cyl but they should both go same way YES??? IAW other members pics. > I did not have a "shim/s like the manual shows on outside of pads so thats weird??? . I used a syringe filled with Bk Fluid to "Blow " all the air pockets out of the master when I did all the Bleeds ( Like 4 now) Even turned it upside down and backwards !!! And yes like 2 12 oz bottles of fluid later that son of a gun is flushed good LOL And only fluid comes out Bleed port

The piston seal?? it had a raised edge on 1 side , I put the raised edge of the square seal oring facing the OUTSIDE of the puck. Is that the item you said gives Spring back ?? Is that the correct orentation or should raised lip be facing on inside section of puk? Seems that I have been able to get better "spring back now like 1/2 to 1 sec return but still not satisfactory to me. I Am an Aircraft Mechanic by Trade so There is ONLY 1 way to do it . The Right way . Wont go on the road till its correct.

Some Background My dad purchased this bike new in sept of 1979 from Yamaha and it is All original except the seat he had 1 custom made 20 plus years ago. Also the wind screen . He did everything by the Shop manual which I have along with a Clymer one also. ****ty Pics of brake parts orentation in both books.

I will tear apart the calipers and M/C if any one can say I did something STUPID or just keep Beeding !!!!!! Also thanks for all the tidbits of info on the SF itself THANK you All