LeMans 1000 (IV) Rear brake (Update: Solved)

[SPScottNT]

Good Morning,
I have a new to me '86 Lemans.  One of the PO's de-integrated (disintegrated?) the rear brake so the hand lever now operates the front two discs and foot pedal operates rear disc.  The front M/C looks stock but I believe it has been bored and sleeved to 15/16mm by the feel of the brake.  The rear M/C feels like it was left stock - 16mm M/C operating a single F08 caliper - rock hard, no travel and no feel.  I've searched the forum and found good options for smaller rear M/C's that fit the earlier bikes with the integrated reservoir.  The LM IV rear M/C has a remote reservoir and different mounting points - is there a smaller rear M/C available that will bolt up and use the same brake pedal linkage?

I've also read how some have used a larger F09 caliper in this situation on earlier bikes where the rear caliper mounts above the rear axle.  The LM IV rear caliper mounts below and forward of the axle, so I don't think the currently available FO9 caliper will mount on the LM IV caliper bracket.  Would a rear caliper bracket from another model, such as a Cal 2 or SP fit the LMIV so the caliper could be mounted above the axle and the F09 be fitted?

Are there other options for improving the brake feel / m/c piston to caliper ratio that I am not considering?  I would not mind reverting back to stock, but the fact that the front lever/MC/throttle and switch gear are an integrated unit (and work well as is now) make me want to leave it alone.    Thanks.

Scott
Athens, GA

[redrider90]

MG cycles carries the F0 9 in stock. They probably could tell you the answer. The F09 was stock  on the rear older model SP IIs I believe. I use an F09 on the front right of my Mille.

[SPScottNT]

Thanks Red.  I need the mirror image of the F09 that MG Cycles has in order for it to mount to the LM IV caliper bracket.  The caliper brackets (and location of the caliper with regard to the axle) are different on the LM IV as compared to the SP which uses the F09 you reference in the rear. 

Scott
Athens, GA   

[Denis]

MG cycles carries the F0 9 in stock. They probably could tell you the answer. The F09 was stock  on the rear older model SP IIs I believe. I use an F09 on the front right of my Mille.

I think all SPIIs used an F08 on the back. The F09 was used on the rear of early SPs and even they might have been replaced by F08s later on.
Unless I have that backwards...

[wirespokes]

I've gone through the same scenario - bike came to me de-linked with a larger front MC. I eventually tried the smaller original front master and like it better.

The rear is just like you say. There aren't many options. One is to use the rear master from a V50 or 65 that was cable operated but the same as ours with a smaller bore. It's NLA.

Other guys have adapted sport bike calipers - available on ebay for twenty or thirty bucks. I got one, but didn't really come up with a clean way of mounting it.

And then I had the idea of lengthening the lever arm. There's room to make it twice as long. Why not? That will double the leverage - just what we want.  I think I'll make a whole new arm, it shouldn't be difficult.

[Dukedesmo]

Don't know if it uses the same type of M/C but I delinked the brakes on my LM2, I used a 15mm Brembo RCS M/C on the front which works great and at the rear I found a M/C of the same design as the original but with a smaller bore as used on many Italian bikes of the era (Ducati, Laverda). IIRC it's a 12mm rather than 15mm.

The only issue is that it was made to be fitted facing the other direction so I had to make some minor modifications for it to fit: swap the counterbore to the other side etc. but it works perfectly.

As supplied;



Modified;



Fitted;



Here's one on ebay;  https://www.ebay.co.uk/itm/Ducati-750-860-bevel-drive-NOS-rear-Brembo-brake-master-cylinder-with-NO-SWITCH/360446989620?hash=item53ec509534:g:FcEAAMXQ56ZR~CQj

[Diploman]

IMO, Dukedesmo's rear MC swap to a 12mm bore unit is by far the simplest and most effective solution to the post-de-integration rear brake dilemma.  You need a pump that will supply less brake fluid to the caliper for a given pedal travel - this it it.

[SPScottNT]

Thank you Duke.  I've read and re-read your previous posts describing your elegant solution.  The LM IV has a different rear master cylinder than what was used on the LM II, III, SP, Cal-2 etc.  So mounting points and fitment are different.  I've got one of the original older M/C's you dealt with on hand and can use it to mock something up to see I can make you LM 2 solution work on the LM 4.  Agree with Diploman - this would be the best solution - just not sure it can be adapted.  Thanks and please keep the suggestions coming.   
 
Scott
Athens, GA

[Antiquar]

I've been having the same issue with a delinked bike. I had no idea one of these smaller bore cylinders existed, definitely going to pick one up. Interesting thread!

[Antietam Classic Cycle]

Maybe have the original master cylinder sleeved down to 12 mm and use this kit?
http://www.mgcycle.com/index.php?main_page=product_info&cPath=110_112&products_id=754

Perhaps this place, not far from me, could help?
http://whitepost.com/brake-sleeving-rebuilding-services/

[jvb]

Depending on your metal working skills this may offer inspiration:
http://hmb-guzzi.de/Lever-foot-brake-cylinder-Agostini-footrest-kit-LM-1-2-3

[pressureangle]

I swapped in the front cable operated M/C from a V50. It's a smaller bore, just swap on the LM lever and it's the same fitment. A much nicer feel-not exactly overpowered, but you can press on it without standing and lock the wheel.

[wrbix]

Don’t have my copy with me at this instant, but Guzziology has a very nice discussion of this issue with a table (as I recall) of ratios and cylinders available.
Amazing what one can answer in Guzziology.

[SPScottNT]

Depending on your metal working skills this may offer inspiration:
http://hmb-guzzi.de/Lever-foot-brake-cylinder-Agostini-footrest-kit-LM-1-2-3

If I understand correctly, the function of this piece is to shorten the distance between the brake pedal rod attachment point and the pivot point on the M/C actuating lever.  It is shortening the lever on the M/C, decreasing leverage, and requiring more foot pedal travel.  Am I understanding this correctly?  The lever on the LM IV is a different shape than this piece, so its not a bolt on, but a similar piece could be made up.  Inspiration indeed.  Thanks.

What I am still not quite able understand centers on the fact that while the mechanical leverage is changed, the fluid volume and M/C piston area to caliper piston area is not changed.  So what could one expect in this situation - more pedal travel but similar firmness or wooden feeling?   
   

[SPScottNT]

I swapped in the front cable operated M/C from a V50. It's a smaller bore, just swap on the LM lever and it's the same fitment. A much nicer feel-not exactly overpowered, but you can press on it without standing and lock the wheel.

This sounds great, although I am hearing this part is NLA.  Will keep my eye out for a used one.  Thank you.

[wirespokes]

This sounds great, although I am hearing this part is NLA.  Will keep my eye out for a used one.  Thank you.

What that part is, is the lever mounted to the master cylinder that pushes on the piston. A longer arm - in this case 10mm longer - will provide more leverage.

The confusion is that there are two systems here - a mechanical one and a hydraulic one. You can change the ratio by reducing the master cylinder size, or you can lengthen the lever arm at the master. A longer lever increases the force applied at the expense of travel. Either way works.

[SPScottNT]

Thank you, Wirespokes.

To summarize and for future readers of this thread: The NLA part is the V50 master cylinder referenced by Pressureangle (original MG part # 1966 0200) which would be a bolt on / no-fitment-modification-required solution for the LM IV.  The aftermarket lever mounted to the master cylinder referenced by JVB seems to be available, but fits the early master cylinder used on the SP, Cal 2, LM III (MG part #2966 7000B).  A similar piece that would fit the LM IV master cylinder (MG part # 3166 6990) could be made up by someone with machining skills.  The larger F09 caliper that is currently available (MG part # 1865 3060) does not fit the LM IV because the LM IV carries its rear caliper ahead and below of the axle.  The available F09 will fit in place of rear F08's carried above or behind the axle (Cal 2 or LM III; SP likely already fitted with the F09).  I think someone with machining skills could modify a Cal 2, SP, or LM III caliper bracket / carrier so that it would fit a LM IV, carry the caliper above the axle, and then utilize the larger F09 caliper to obtain a better m/c piston to caliper piston ratio.  Lastly, Charlie's suggestion of having the original rear master cylinder re-sleeved to a smaller size (11mm, 12mm, 13mm) and installing components from an appropriately sized rebuild kit would also be a possibility - one might need to verify that the rebuild kit piston length would work in the re-sleeved m/c barrel.  I appreciate all of the input and will post again if I arrive at a solution for myself.  Thanks.

Scott
Athens, GA     

[pressureangle]

What that part is, is the lever mounted to the master cylinder that pushes on the piston. A longer arm - in this case 10mm longer - will provide more leverage.

The confusion is that there are two systems here - a mechanical one and a hydraulic one. You can change the ratio by reducing the master cylinder size, or you can lengthen the lever arm at the master. A longer lever increases the force applied at the expense of travel. Either way works.

Wut?

The bore on the v50 MC is smaller. The lever is the stock LMIV lever on the v50 cyl.

[Diploman]

Scott, I have a V50 cable-operated MC which formerly operated the right front brake on my 1980 V50 II.  It worked well, but the cable actuation (emulating BMW of the era) gave the brake a dead, wooden feeling, with no feedback.  I replaced the OEM MC (Brembo part no. 10.2934.00) with a handlebar-mounted Frando radial MC which has much better sensitivity.

Although bore size is not marked on the unit, according to Dave Richardson this front MC is 12mm.  It was working fine at the time I removed it from the V50, but it's possible it might need a rebuild kit after sitting unused for a number of years.  It requires a remote reservoir, connected by a hose to the MC.  If this - now NLA - item might be useful in solving your rear brake problem, I'm not planning to re-install it, so it is available.  PM me if interested.

Will

[SPScottNT]

PM sent.  Thanks!

[wirespokes]

I've got a master cylinder rebuild kit for that V50 master cylinder. I was anticipating sleeving mine down to 12mm. I've decided to lengthen the lever arm instead.

However, if anyone else has a V50 rear master they'd like to sell, I may have a use for this rebuild kit. Otherwise, I'd like to see someone else get some use from it.

[SPScottNT]

I installed the aforementioned and pictured master cylinder from Diploman.  The two master cylinders are nearly identical except for the actuating levers (foot control clevis vs cable pull) and the fact that the V50 cylinder is just a little shorter aft of the rear mounting point than the LM1000 master cylinder.  This worked in my favor in that it leaves room to install a brake light switch banjo bolt, which extends further back than a regular banjo bolt.  The actuating levers interchange and must be swapped for this application.  Also, some short wire leads need to be made up to connect the new brake switch to the old brake switch leads.  No other fitment or fabrication required.  Feel at the pedal with this set up is about the same as the feel at the pedal on a stock linked system with two F08 calipers.  Much, much improved over how the brake felt and behaved with the large MC and single caliper.  Thanks for all of the help.

Scott
Athens, GA   

[Diploman]

Glad it worked out for you, Scott.  Safe travels!
Will