Rear Brakes....again

[Toolmaker Tim]

This is for all the XJ experts. The local Dr Frankenstein that put my bike together used a (I think) XJ master cylinder. It has 2 connections for the brake line. I haveb't been able to get good braking power with the rear brake. I know the XJ had a kind of anti-lock brake system. My question is; is there a difference in the amount of avalible pressure to the caliper depending on which port you use?

[Crazcnuk]

The Xj doesn't have any sort of anti-lock.

What they did is made the rear pedal actuate one front caliper and the rear caliper.

The front brake lever only operates one front caliper.

I would assume, if they blocked one of the ports i would guess you run out of lever travel when the blocked port pressures up. Since this circuit is very short it may pressure up too soon.

I don't know if the ports are metered differently or not. I would think your better of getting an XS MC and going from there.

Is your pedal mushy or hard?

[TopCatGr58]

Hey Tim,

The XJ rear MC does have a metering system, and sends more pressure to the front than the rear, so IF you were able to cap off the rear port, and connect to the front port for your rear caliper, you would probably find more pressure available to it. Otherwise, like stated, getting an XS rear MC would be the next thing!
T.C.

[CatatonicBug]

That rear MC is the same as what was used on the '81 XS MNS. The MNS has a 11/16" bore, and the XJ has a 3/4" bore (printed on the side of the MC). I guess they did that as a trial run, then used it on the XJ. There is a "Proportioning valve" connected to the rear MC, which has the 2 ports coming out of it. As long as one of the ports is blocked, all of the force automatically gets pushed out of the second port, no matter which port is plugged. The Proportioning valve is really just a funny rubber seal with bumps on it that allows a metered percentage of pressure to pass through the second port, but only works if the system is properly connected and bled. If you're using only one of the ports, you're probably better off switching to a proper XS master cylinder, since the larger bore may cause the rear caliper to lock up more easily if only one port is used.

[Toolmaker Tim]

Rear Brakes

In reguards to the first question, the brake pedal is hard. Have taken it apart and cleaned the M/C and the caliper. New brake fluid and bled. The problem is I cannot make the back tire skid short of shoving a 2x4 thru the spokes. The brake line is hooked up to the front port. I guess I need a new rear M/C. Anyone have one cheap? I can't afford much because I'm laid off now and things don't look like they are getting any better at the shop.

[renegade_xs11g]

I guess the biggest question I have on this is: How close to skidding is it?
does it feel like you're right there? or not even close?

A few more: are you still running rubber lines? If so, how old are they?
the rotor....can it be turned? has it been? within spec?

I could go on, but there are a lot of things to think about....

[renegade_xs11g]

I know I have a rear MC, came out of my SG, but I'll need to find it.....

[DGXSER]

Anyone who knows where to get a bike rotor turned short of a machine shop let me know. I have one or two that could be turned I think, but no one in town does or knows anyone who will turn a bike rotor.

[Toolmaker Tim]

!$&**@# Brakes

Hey renegade, I would venture to say that I'm not even close. Yes they are rubber hoses, but I should still be able to get a mostly worn Cheng S##t tire to slide a little. Short of carrying around the before mentioned 2x4 for extra stopping power.....
I still think the problem is the M/C. When the pedal goes down it is rock hard, but you are still sailing along. Yes I know to use both front and back brakes, but the back one not being right is bugging me. Disc is in good shape, new pads, everything in the rear caliper looks good, the M/C isn't the proper one and until I prove it isn't the problem, I think it is. I rebuilt the front calipers and M/C at the same time and they work well. Well, cleaned them up anyway and put in new fluid(DOT3).

[Toolmaker Tim]

Turning rotors

DGXSER, I have a spare set I was going to try refacing, but I kept puting it off and then I was laid off. I don't have access to a lathe or grinder now, so my little project is on hold. They won't let me use the equipment because of insurance reasons. Guess I wait. Maybe I can talk them into letting me us the lathe. However I have heard that the proper way is to grind them. Anybody know for sure?

[Crazcnuk]

What would happen if he got a Y fitting and used both ports to run the back wheel?

[CatatonicBug]

Tim - If you take the rear caliper off the bike, but leave it connected to the MC, can you move the pedal and watch the piston move in the caliper? You should technically be able to pump the fluid into the caliper and force the piston out of the caliper. It sounds like something is frozen in your system. When you took the MC apart, did you open up the Proportioning valve and clean in there too? Make sure the MC is pumping fluid when you hold it in your hand, and that when you press in on the piston, you can't hold back the fluid from shooting out the holes. Did you bench bleed the MC before you put it back on the bike?

I'm not sure why you want to skid the tire in the first place. I can understand wanting to be able to stop, but locking up the tire is not a good thing.

[Toolmaker Tim]

Skidding

It isn't that I want to slide the back tire or anything, it's just that the brakes should be capable of doing that. If they can't, something is wrong. Yes the piston moves and the proportioning valve was cleaned out. SIGH.... Iguess I better find a 2x4 to my liking.

[xs11lover]

When you start to push the back brake down, how far does it travel before any presure is applied to the disk? And why do you have an xj master if you have a xs sf? I'm lost. The first day I had my sf I skidded the back wheel, right down rt 14. It worked a little better than it should.
You could always sell your xj master on ebay. I think they are harder to get than xs's.

[TopCatGr58]

Hey DGXser,

A new user posted where he used the Navy mechanic centers brake lathe to smooth his out, but he had solid not slotted rotors, and he said the cutting tool was toast by the time he got done! Tim has stated because of the extra hardness of the rotor metal, and the slots, that a carbide lathe tool would probably break, or there would be too much chatter to effectively get a smooth cut, hence the need for a grinding type rig instead. It's been stated before, that a machine shops flat grinder is what would be needed for these, however, not sure how they would get the other side where the recess is!? And then there's the question of usable thickness limit, due to chances of overheating, warping, etc.!

Hey Tim, did you check the piston linkage where the pedal pushes it up into the MC? Did you adjust the piston so that it's very close and almost starting to push the MC piston in when the pedal is all the way up! Perhaps you are running out of pedal travel before you run out of piston travel???
T.C.