1980 BMW R100T Brake Upgrade Question

[nevadaslim]

I'm doing a restoration on a 1980 R100T I bought on e-Bay.

One of my projects was to move the brake master cylinder from under the gas tank to the handlebars(all BMW parts except for the brake lines).
Today a friend and I filled and bled the brakes for the first time. I'm using Motul RBF 600 brake fluid, and Speed-Bleed nipples to bleed the brake
system.

We appear to have gotten all of the air out and there are no leaks. But the brake lever is still soft/spongy. I've bled both calipers at least (4) times just
to make sure all the air is gone.

I can't really test the brakes since the bike still isn't running. But I can spin the front wheel while on the centerstand and the wheel
stops instantly! So my brakes grab, but like I said, the pull on the brake lever is soft compared to the Japanese bikes I've owned.

My question is, is a soft pull normal for this configuration, handlebar mounted master cylinder and ATE calipers? I've attached photos of the
installation in case someone can point-out something wrong.

I should also mention that the brake system sat empty(w/o fluid), both of the rebuilt calipers and the master cylinder for about a year while I was
working on other parts of my bike. Could that have caused a problem? Perhaps dried rubber seals, O-rings. Maybe sitting over night they'll moisten up?

Thanks for any ideas, suggestions, ect...

View attachment 51642,
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View attachment 51648

 

[lkchris]

The diameter of the master cylinder piston is matched to the calipers (caliber is a size measurement for guns). There are several versions of the Brembo master cylinders as, if nothing else, some bikes have 2 front calipers and some just one. Nothing says the fluid space behind the caliper pistons is the same comparing the ATE and Brembo calipers, either. Soft isn't normal. Somewhere somebody has worked out the best version of the Brembo master cylinder to use.

 

[nevadaslim]

The diameter of the master cylinder piston is matched to the calipers (caliber is a size measurement for guns). There are several versions of the Brembo master cylinders as, if nothing else, some bikes have 2 front calipers and some just one. Nothing says the fluid space behind the caliper pistons is the same comparing the ATE and Brembo calipers, either. Soft isn't normal. Somewhere somebody has worked out the best version of the Brembo master cylinder to use.

As for now, I have a brand-new 13mm BMW master cylinder and rebuilt ATE calipers, from what I understand they should work together well.

 

[toooldtocare]

I noticed that you added floating rotators. When this was done did you reset the pads? The caliper rotates around a pin in the fork leg. One of the pads is flat, the other is tapered. As they wear the caliper rotates on the pin. The adjustment is rotating the pin in the fork to move the location of the caliper. Once centered is is locked into place. If it is not correct, the pads will not be making full contact, thus giving you a spongy lever.

See here. http://forums.bmwmoa.org/showthread.php?61035-75-R75-6-caliper-adjustment

 

[nevadaslim]

I noticed that you added floating rotators. When this was done did you reset the pads? The caliper rotates around a pin in the fork leg. One of the pads is flat, the other is tapered. As they wear the caliper rotates on the pin. The adjustment is rotating the pin in the fork to move the location of the caliper. Once centered is is locked into place. If it is not correct, the pads will not be making full contact, thus giving you a spongy lever.

See here. http://forums.bmwmoa.org/showthread.php?61035-75-R75-6-caliper-adjustment

Thanks for the reply. Along with the new EBC rotors I also replaced the pads, and did center the calipers. But... I may have missed something, so I'll definitely check-out your link.
That would be an easy fix.

Thanks again,
Mike...

 

[ccolwell]

It sounds like there is still some air somewhere. The most likely place is at the brake line to under tank splitter junction or in the MC at the brake line junction (I'd bet the first as it's pointed uphill, as it were). For the first, try dismounting the junction and turning it to face downhill and rebleed. For the second, cover all the paint work, wrap a rag around the junction, loosen the bolt and tighten to just past finger tight. Then apply light pressure to the lever and loosen slooowly until just a bit of fluid comes out and retighten while holding pressure on (this step should be done rather briskly). If that fails, I'd dump the speed bleeds -- they have worked fine for me on one or two applications but they always seem to have slightly mismatched threads, so I gave them up. Besides, the Speed Bleed bag they sell makes doing it the 'old' way very easy.

The last option is to use two brake lines directly from the MC to the calipers. You'd need to find a brake switch fitted to a banjo bolt but I'm sure there's one out there somewhere.