74 850t front brake

[Mc Tool]

Hi team Ive been fiddling away at this old bike for a while now , and have come to a stop , better than the bike , thus the post
The bike has a 300mm brembo disc ( that is flat , round and not really worn at all ) , a F09 caliper ( freshly rekitted ) and new brembo pads and a braided line , all good so far , the master cylinder is the issue , Its not the original ,I dont even know what its off , a honda maybe and it leaks. The brake is very wooden , no feel to it and requires a real good squeeze and will lock up without warning . Im sure the m/cyl not the right size, but I have been unable to find out the correct size ( not in the book ) . Can anyone advise of the correct sized Brembo master cyl ?And , Im giving some thought to fitting another disc. I have a pair of F08's that will do the job, question is is it worth it ?... and what size master cyl would I use .What Im looking for is more feel , less lever effort required and Im thinking that fitting the correct master cyl will go a long way to solving that problem but if anyone has done the conversion I'd like your opinion. The bike is not ridden hard , mostly 2 up ,non linked brakes and I have a buggered right hip/leg/foot and using the rear (2ls drum) brake is something that takes a bit of forward planning so the front brake tends to get all the work.
Any comments / advice appreciated
Hamish

 

[guzzigirl]

I have a 1975 850T which had twin front disks when I bought it second hand. They don't match so I don't know if they were fitted aftermarket or not, although I have heard that it could have come from the factory like that. I would recommend the second disk. I only use the back brake for stabilizing. As for the master cylinder, mine was replaced with a smaller one than the original as it seemed to give a better feel to it so going back to the original might not be your best solution. A leaky cylinder will definitely affect the feel too. I'm sorry I can't tell you any further details about what master cylinder I have as it was a long time ago. Hope this is helpful.

 

[ARVENO]

are you sure your brembo caliper is a F9 ?
It should be F8 front .
The F9 "usually " is on the r ear.

With single disc , the master cylinder you need is 12mm.
check this one out

http://www.mgcycle.com/product_info.php ... ts_id=3107

BTW a brand new master cylinder is cheaper than medical bills..... i would get a new one though.

good luck
Marco

 

[Rafael]

Here's the coffin res model for two calipers. http://www.mgcycle.com/product_info.php?products_id=81

If you have an F09 in the front, a PO swapped it out. That unit came on the SP rear and maybe the G5. I think some beemer models had it on the front.

Here's the new disk, http://www.mgcycle.com/product_info.php?products_id=1029 $50 more than the last unit - oh well. But at that price you might want to go with a SS floater unit. http://www.mgcycle.com/product_info.php?products_id=2652

 

[contractor20]

I've got one on my T3 single disc front and it leaks a bit but I dont worry too much. It's been on there since a drop in 1978 and I got it S/H from a wreckers! They have those nice alloy spin on caps. It is a bit wooden (read very) but only use it to supplement the linked foot brake.

The jap M/Cylinders were typically 16mm.ie 5/8" ( usually marked underneath somewhere ) and would probably be ok for a twin disc setup and you should be able to get parts for them. If you want a set up with slightly more feel try this one : http://www.mgcycle.com/product_info.php?products_id=81

If you stay with the single disc try this : http://www.mgcycle.com/product_info.php?products_id=1037. Or for the authentic look this : http://www.mgcycle.com/product_info.php?products_id=3107 plus lever

B)

 

[Trout]

Hey McTool!
Converting to dual disc brakes will be quite satisfactory & worth the time & money. I can stop old Red Dog faster than many new bikes & it is a T with a rear drum. Like GuzzGirl I use the rear for slowing up or "stabilizing".

The lock up is probably the master cylinder not allowing fluid to return to the reservoir; providing that the caliper was rebuilt correctly. Replace it with a new one from MG Cycle. To convert it to dual disc you'll need to scrounge another disc, longer bolts, a splitter, a caliper with mounting bolts & a hose.

If you get the new master and put it on with just the single disc you can be riding with decent brakes while scrounging extra parts, they're out there you just got to find them.

Trout

 

[Rafael]

Replace it with a new one from MG Cycle. To convert it to dual disc you'll need to scrounge another disc, longer bolts, a splitter, a caliper with mounting bolts & a hose.

AND a new bearing carrier, which are still available from MG Cycle http://www.mgcycle.com/product_info.php?products_id=579 I'd get rid of the F09 and get two F08s to make sure both sides are the same. You can get these brand new complete with pads.

 

[Mc Tool]

Its definatly an F09 thats on there now , and I have a pair ( 2 actually ) of F08's ( yeah Rafael you didnt actually need to say to get 2 F08's so both sides are the same ) and yes the bearing carrier ( was planning to make one ) and of course an extra disc ( yes Rafael I was planning to use 2 disc's......... the same ). The locking I mentioned is just down to lack of feel , and possibly the skinko tyre, and another part of the problem is probly me being used to a set of 320mm 4 pot brembo's .
Thanks for the comments/advise
Hamish

 

[Rafael]

bearing carrier ( was planning to make one )

I love it! Photos when you're done please. Oh, and two new bearings :silly:

used to a set of 320mm 4 pot brembo's .

Well, you can have them again, by making a set of adapters for the 4 pots - would work pretty good with the EBC floaters.

 

[Mc Tool]

Truth ... I was going to get a friend to make the carrier , but now that I see I can buy one ... probly do that , spent a bit of time lookin for a used one (round here ) .
Big brakes would be cool but I want to keep the thing looking the way it should and besides , it you do "big brakes " where do you stop . There are a few imposters on this bike already ( pix if you want ) , GN 250 blinkers , chinese "norton " mufflers, Yamaha AG 175 tail light and carrier , yam xz550 switches , home made wiring harness ( totally different from stock ) ,dyna ign/coils, lucas ACR rect , bosch reg and the star of the show ..... the speedo/tacho nacelles are actually sewer pipe , but mostly nothing looks wildly out of place and most people dont notice at all

 

[manxkipper]

The wooden feeling and sudden lock up is probably due to the ratio between the M/C and the caliper being incorrect. In order for the F09 (48mm) caliper to work with "feel" you should have a M/C with a diameter of 13mm. A 14mm unit will do but will have less feel. The way to work it out is to calculate the area of the various circles i.e. the m/c and the f09 (both piston areas added together) to get the ratio. Ideal is between 23 and 27 to 1. The lower the ratio the less the feel at the handlebar. The standard tonti m/c for the front brake is 1/2" which will give a ratio of 28.5 to 1.