[Gary Huston]

Hi guys, I'm rebuilding / customizing a stolen recovered 79 T140 and I need to replace the rear master cylinder. Does anyone know of a suitable replacement that would be upto the job? I don't care about it's origin so anything considered as I can make new brackets. Just thought there might be something available from a modern machine that wasn't as bulky as the stock item.

 

[Dangerous Doug]

I got stainless steel replacements from the Bonneville Shop. Also replaced the rubber hoses with braided lines. Much better than stock but still nowhere near modern brakes

 

[StuartMac]

Hi Gary,

Firstly, welcome to the Forum.

79 T140
replace the rear master cylinder. Does anyone know of a suitable replacement that would be upto the job? I don't care about it's origin

Regrettably, replacing with just some random master cylinder is unwise. Original was/is a relatively-large 5/8" ID and, thanks to the stupid standard lever length (dictated by the standard footrest position), it's already very easy to lock the rear wheel under braking.

You can obviously reduce the lever length with rearsets but ime that only makes the standard master cylinder-caliper ratio just acceptable, smaller-ID master cylinder would bring back the problems. Rear disc was only because Everyone Else Did It at the time, few years later, Meriden made a drum to fit in place of disc-'n'-caliper on some versions.

So ime and mho, unless you also intend to replace the caliper with either the drum or something with a sensible ratio to match your chosen master cylinder, stick with the standard master cylinder (albeit stainless as Doug posted)?

Hth.

Regards,

 

[wol]

As already advised - its best to stay with the standard set up, but upgrade to a stainless steel replacement cylinder -- i have adapted a master cylinder from a Hinkley Tiger matched to its dedicated brake lever - but this was as part of changing to "rear-set" footrests - the M/C is mounted on the outside of the frame - the whole job took a lot of time
-- the original set up looks far neater (you cant see it) - unfortunately its in such a stupid position and gets covered in road crap

 

[wol]

If you can find a pic of the TSS AV model and zoom in on the brake pedal area you can see an alternative Meriden master cylinder position -- this used the standard length brake pedal but with an additional short arm that connected to a rod that acted on the Master Cylinder which was bolted to the frame tube -- that M/C is the same as fitted to some BMW bikes - dont know the model but it is 5/8 bore -- you would need to weld some lugs to the frame for the m/c and modify the pedal pivot fixing

 

[GrandPaulZ]

If you have decent fabrication skills, I'd go visit a local m/c salvage shop and see what I can find. I've seen umpteen different ways to retrofit a modern master cylinder to a classic Triumph, and have never heard follow-up reports of rear wheel lock up.

Direct, intermediate levered, cable operated lever, vertical, horizontal, etc.

Are none of them talking about it or admitting it? Your guess...

 

[StuartMac]

Hi Gary,

If you can find a pic of the TSS AV model and zoom in on the brake pedal area you can see an alternative Meriden master cylinder position

Triumph TSS AV; if you scroll down the page, third photo. down, cursor over it, photo. enlarges when the mouse wheel's rolled forward. The bike also appears(?) to have the rearset footrests Meriden developed but never sold.

Hth.

Regards,

 

[Rancidpegwoman]

Hi Gary,
I am replacing a standard rear master cylinder with a Lockheed racing version at the moment. This is because I am changing the swingarm and there is no longer room to accommodate the master cylinder in it’s standard tucked away position.
It is just heartache making the modification, if you choose an easy position it looks terrible, like a huge carbuncle on the side of the bike. If you choose a discreet position then the transfer mechanism is complicated and difficult to manufacture. With the large 44mm caliper and long brake lever fitted (see Stuarts previous post) getting the master cylinder/caliper ratio correct is difficult. You can end up with no power, or power with no control.
The standard stainless steel replacements are cheap now and most importantly you can have it fitted in 45 minutes.
The alternative is weeks of cutting, grinding, welding and machining to get an acceptable result.
standard looks very attractive to me.

regards
Peg

 

[Gary Huston]

Thanks for all your replies. Fabrication / engineering / welding is my game so that side is a piece of cake! I have done away with the standard huge brake pedal as I have moved the foot pegs back more under my arse. I think I will be going with the BMW r100 cylinder, it has a weird linkage but can be hidden behind the down tube and the reservoir hidden under the seat.

 

[Gary Huston]

If you have decent fabrication skills, I'd go visit a local m/c salvage shop and see what I can find. I've seen umpteen different ways to retrofit a modern master cylinder to a classic Triumph, and have never heard follow-up reports of rear wheel lock up.

Direct, intermediate levered, cable operated lever, vertical, horizontal, etc.

Are none of them talking about it or admitting it? Your guess...

I only wish we had motorcycle salvage yards in abundance, unfortunately they are like hens teeth around here! I have picked up a few sample items from a biker friend with a shed full of bits so I can have a play around with ideas. I must admit I was hoping for a few ideas from the adventurous bike builder types rather than the cautious weekend tinkerers! No offense intended!

 

[GrandPaulZ]

I was hoping for a few ideas from the adventurous bike builder types rather than the cautious weekend tinkerers!

I cleaned it up a bit after this.

It was just a matter of picturing in my mind's eye, what pieces COULD POSSIBLY work, then making them work.

I had 4 different master cylinders and a couple of different brackets (you can see the spare unused mounting hole just aft of the footpeg)

 

[Gary Huston]

View attachment 747658

I cleaned it up a bit after this.

It was just a matter of picturing in my mind's eye, what pieces COULD POSSIBLY work, then making them work.

I had 4 different master cylinders and a couple of different brackets (you can see the spare unused mounting hole just aft of the footpeg)

I’m going to do exactly that, get a handf of parts and mix and match until I get what I’m looking for! Thanks

 

[Rancidpegwoman]

Thanks for all your replies. Fabrication / engineering / welding is my game so that side is a piece of cake! I have done away with the standard huge brake pedal as I have moved the foot pegs back more under my arse. I think I will be going with the BMW r100 cylinder, it has a weird linkage but can be hidden behind the down tube and the reservoir hidden under the seat.

Hi Gary,
Do you have an estimate of the reduced LR compared to standard , after you moved the footpegs. The original BMW caliper most likely had a piston csa of 2268 mm2, where as the Lockheed caliper (if retained) has a csa of 2640mm2.
My fear is that the pre 81 BMW master cylinder generating a hydraulic ratio of 15:1might not be the best choice to compensate for the lowered mechanical advantage, the post 81 BMW master cylinder fares a little better at 17.25:1 but not much. This coupled with the poorly designed relief ports on the BMW master cylinder, leads me to believe that you could make a better choice for a replacement.
Restoration of mechanical advantage through the linkage mechanism will help return some power to the brake, but you have to be careful not to introduce too much ‘woodeness’ to the brake; having power without control can be difficult, especially on the rear brake (Triumph/Lockheed did not get it right). You might find it useful to get better power and importantly control of the brake by careful selection of the hydraulic ratio, once this is in place then the mechanical ratio can be adjusted to suit your new pedal arrangement.

regards
Peg.

 

[Gary Huston]

Hi Gary,
Do you have an estimate of the reduced LR compared to standard , after you moved the footpegs. The original BMW caliper most likely had a piston csa of 2268 mm2, where as the Lockheed caliper (if retained) has a csa of 3041mm2.
My fear is that the pre 81 BMW master cylinder generating a hydraulic ratio of 17.2:1 might not be the best choice to compensate for the lowered mechanical advantage, the post 81 BMW master cylinder fares a little better at 19.8:1, but not much. This coupled with the poorly designed relief ports on the BMW master cylinder, leads me to believe that you could make a better choice for a replacement.
Restoration of mechanical advantage through the linkage mechanism will help return some power to the brake, but you have to be careful not to introduce too much ‘woodeness’ to the brake; having power without control can be difficult, especially on the rear brake (Triumph/Lockheed did not get it right). You might find it useful to get better power and importantly control of the brake by careful selection of the hydraulic ratio, once this is in place then the mechanical ratio can be adjusted to suit your new pedal arrangement.

regards
Peg.

Peg, thanks for your input but I’m afraid I have no idea of figures, I’m a suck it and see sort of builder. I was starting with the BMW cylinder as it’s roughly the same bore as the original and I know someone who has used it successfully. What would you suggest as a better replacement?

 

[Rancidpegwoman]

Hi Gary,
How would I know what would work, us "weekend tinkerers"are only concerned with making sure our crash helmets, Tanks, Boots and handbags match each other.
I’m joking of course.
Sorry: I tried but I just could not resist it 😊
———————————————————-
The ratios are calculated from the cross section areas of the pistons, not the diameters, because there is an exponential element in the calculation, a small change in the diameter will make a large difference in the ratio.

A 13mm diameter master cylinder will give you a 23:1 ratio, an imperial 1/2" diameter will give 25:1, 12mm diameter master cylinder will give 27:1.
I like to have between 22:1 and 26:1 for feel.
However this does not match the original long lever, a standard rear brake will be locking up all of the time with a small diameter master cylinder.
Even the standard 5/8" diameter master cylinder (15:1) locks up very easily with the very long standard lever.

Reducing the diameter of the master cylinder is only viable if you have already reduced the mechanical advantage of the lever, buy fitting rear sets or moving the footrests back.

If it was my bike I would look for a master cylinder of 13mm, these are relatively common, this will add some power to the rear brake and better ‘feel’ (control) over the standard setup.
Then I would experiment by a mechanical lever system that can be adjusted, or quickly adapted. I would then try to build an input lever length to operate the master cylinder that allows me to lock the rear wheel, but I would have to work hard at it. If the wheel locked too easily I would reduce the mechanical ratio, if I could not lock the wheel at all then I would increase the mechanical ratio.
You should end up after a few experiments with a brake that is reasonably powerful, but is easy to control, giving good feedback to your foot.

Good Luck
Post some photos when you are done.
Regards
Peg.