Brakes won't fully pressurize

[eman13261]

To begin with I understand that there are a lot of threads concerning brakes, however i'm posting because i've been attempting to fix this issue for about a month with little to no progress and have reached despiration.

Both the front (dual disc) and rear (disc) brakes on my 81 xs650 special II kind-of work and i'm comfortable riding the bike for a short distance but they aren't nearly as responsive as they used to be. I've tried every method of bleeding (vacuum, pre-filling the lines with fluid, reverse bleeding, traditional, ect...), even took it to a trusted shop and they were unable to get the brakes back to their prior performance. Both myself and the mechanic checked the caliper parts, brake lines and master cylinder but didn't find anything abnormal. We came to the conclusion that the next step is likely replacing parts. Any thoughts/input?

 

[retiredgentleman]

Is the lever/pedal rock hard or do they fade away as you apply them?
Have the calipers been taken apart and fully cleaned? Piston O-ring groove clean and O-ring rubber in good condition?
If you still have the old original rubber brake lines, you need to replace them with new lines.
Pads can become glazed..................may need to be replaced.

 

[eman13261]

The front lever will touch the handlebar and the rear brake lever will touch the exhaust pretty easily. Never used to do that. Calipers have been fully cleaned every season (including this one) and all the o-rings are in good condition. My brake lines aren't original but the lower lines for the fronts are rubber while the upper portion and rear brake lines are stainless. Pads were replaced less than ~1500 miles ago and are in good condition.

 

[retiredgentleman]

I recommend you re-build the master cylinders...........................thorough cleaning and new sealing rings.

 

[gggGary]

Both of RG's posts, cleaning calipers means the pistons are out and you have removed and cleaned behind the seals. Without that step the pistons do not move on the seals and retract from the rotor every time you let go of the handle. Also the slide pins must be clean free and lubed. What size master cylinder with your dual disk set up? Just did period piece, from sitting in a field 5 years no brake pressure front brake, to working great , once everything was fully stripped, cleaned reassembled with new pads, bleeding took about 15 minutes. Final step is always a completely uphill path from caliper through reservoir and let it sit 15 20 minutes.

 

[Paul Sutton]

The back brake has a stainless hydraulic line, so if everything is mechanically sound then when you apply it the foot lever should come up against a solid stop. If this is not the case then I think there are three possibilities:

1. Air in the line - But you are confident this is not the case and a mechanic has been involved. Also you have worked on this system in the past without difficulty.
2. Leaking from the system - Are there any signs of leakage around o-rings/seals and banjos?
3. There is a possibility that leakage may be occurring past the master cylinder piston which brings as to Retiredgentleman's suggestion above to rebuild.

The front brake has more variables because of the mixed hydraulic line construction and I think you indicate the presence of two calipers. I personally would concentrate on the back brakes first.

Before stripping them down perhaps you can remove the rear wheel and then try positioning the caliper at different angles to see if any stubborn air bubbles come out. But remember to place something in the caliper to stop the piston getting popped out - perhaps a piece of plywood or an old plastic kitchen chopping board about 8mm. If no success then check the master cylinder. Behind the plastic/rubber piston there may be a small set of reed valves that kind of look like a flower. Sometimes the little petal like valves do break off and allow leakage. You will need to examine everything very carefully. If the master cylinder has deposits or scouring then you can polish it off with metal polish on a cloth wrapped around a pencil (I had to polish mine).

Best of luck and take your time to check every single component.

 

[gggGary]

Like Paul mentions; several had to remove the caliper and position it so there is an all uphill path to the MC, like above for the fronts. For the front if needed loosen the handle bar or remove the MC to get the angle. On most bikes; on the side stand, bars turned full left will do it. On the occasional stubborn bleed I'll remove the caliper and retract the piston into the caliper with a c-clamp and pump it back out with the lever, watch your reservoir so it doesn't overflow or get sucked dry while doing this, repeat if needed. You can check your lever action against the c-clamp.

 

[fredintoon]

Hi eman,
did the dodgy brakes just happen or was it after you did something?
Spongy brakes is air in the system and although others have had no trouble I have found XS-series brakes are a right swine to bleed.
Hard brakes that don't grip as well as they used to can be oil &/or brake fluid on the disks.
And are you sure your bike is an '81 Special II? The photos in Skull's I D sticky show Special IIs with a drum rear.

 

[timbeck]

Good catch by fredintoon on the drum/disk question , 1981 Special 11.
tim

 

[eman13261]

Thanks for the replies I'll try to address them all at once.

Before this issue happened I took apart the calipers and cleaned them (yes removing seals and cleaning behind them as well). It's something I do once/twice a season and I've never had an issue before as the brakes have always been very easy to bleed afterwards.

There are no signs of leaking and I use Teflon tape around the bleeder valve as an extra precaution.

I'm positive it's an 1981 special II but I believe the rear disc was a mod by the previous owner.

What baffles me is that the brakes DO work but not as well as beforehand. I was ways impressed at the stopping abilities of my bike despite the age. I think the rear brake will be my focus and then the front MC rebuild is going to be my first option. Following that I'll look into installing new front stainless lines.

All issues point to an air bubble but I honestly think that option has been ruled out. Could it be possible that dirt has gotten in the line (s) and just isn't being pushed through the bleeder valve?

 

[retiredgentleman]

You could try this simple test, if you can find a bolt of the correct thread, that is the same as the banjo bolt, on the front M/C. Screw in the bolt to block the M/C outlet. Maybe use teflon tape if needed to be sure there is no leakage. With fluid in the M/C at normal level, if you squeeze the lever it should be rock hard. If you can pull the lever to the handlebar, then you know for sure the M/C needs to be cleaned and re-built with a new piston seal.

 

[MaxPete]

RGs remark about glazing caught my eye. I wonder if your pads or the rotors - or both, have become glazed.

I think you can clean them by simply sanding the pads on a flat surface and scuff up the rotors with some sandpaper. .

 

[fredintoon]

- - - I'm positive it's an 1981 special II but I believe the rear disc was a mod by the previous owner. - - -

Hi eman,
all you can truly be sure of with your used bike is that the frame is from a 1981 Special II.
Everything else may have been swapped in from another year &/or model XS650.
For instance, my son's Canadian model 1980 Special came with and a 16" rear disk brake and artillery wheels.
We swapped those wheels for 1976(ish) wire wheels just for looks which left his bike with an 18" drum brake rear.
The only thing that stopped me from swapping his disk rear into my drum rear Heritage Special was the hassle involved
in attaching the disk rear's master cylinder onto my bike's drum rear frame.
Which exact mounting adaptation must have been done by your dreaded PO.
Got a photo?

 

[eman13261]

Hmm that's a good idea regarding the mc test I will definitely try that as well as sanding down my pads.

I have mag wheels on my 81 and uploaded the best picture I can get of the setup. I have seen many special II's with the same adaption which originally made me think it was stock

 

[5twins]

I think someone just hung Special II sidecovers on your bike, lol. All the disc mounting set-up looks factory original. Your footpeg brackets are chrome. They would be black on a Special II. How about your speedo and tach housings? They would be chrome on a Special, black on a Special II.

 

[eman13261]

They used to be black but were worn so I replaced them. The speed/tach housings are chrome but I've seen plenty of special II's with chrome. Additionally my model and engine # correspond with a special II.

 

[Paul Sutton]

I tested my MC as per Retiredgentleman's instruction above after I rebuilt it. The thread is 10mm x 1.25mm pitch. These are not a standard bolt size in the UK but on my 81 SH this bolt size is used to secure the headers to the frame just below the foot pegs. You could borrow one from this same location on yours, just make sure the threads are not damaged or they will damage the MC thread. If you have a spare banjo bolt you could also plug it with epoxy first.

 

[fredintoon]

They used to be black but were worn so I replaced them. The speed/tach housings are chrome but I've seen plenty of special II's with chrome. Additionally my model and engine # correspond with a special II.

Hi eman,
We (or at least, I) are not slagging your bike and like you say, it's got matching Special II ID #s on it.
The drum brake to disk brake rear end is a bolt-in swap, just like the chrome vs black gauges, pegs etc.
And your bumstop seat and long megaphone mufflers ain't stock neither.
However, all those part swaps and modifications individualize the bike to make it uniquely yours.
And that's a good thing.

 

[5twins]

The welded tabs on the frame for the MC mount are not a bolt-on, and his look factory. If not, they're very well done.

 

[fredintoon]

The welded tabs on the frame for the MC mount are not a bolt-on, and his look factory. If not, they're very well done.

Hi 5T,
It was my drum rear Heritage not having those frame mounts led me to abandoning it's rear disk conversion.
And they'd have to be positioned exactly right for the M/C to work in conjunction with it's operating mechanism.
But somebody must have done it?
Is our data incomplete and Yamaha DID make disk rear Special IIs or are there some gifted
amateurs out there fixing them up?