Master cylinder.

[phil13]

Hi all,
         My rear master cylinder seal on my trike has just gone pop, and although pumps fluid as soon as there is resistance it just bypasses in the bore. The question is, it is fitted with a vertical m/c cylinder and in the next few weeks I am going to fit disc brakes . What  master cylinder would you guys recommend to operate the vw callipers ? Or would a direct replacement for my old one be man enough? It's a girling 412+
Many thanks phil.

[Olds]

Not heard of "412+" If the existing setup worked before then it should work with discs. They usually require less fluid movement because the pads move so little, compared to the shoes in a drum brake.
Will the master cylinder only be used for the rear brakes?
You may be able to use a smaller bore master cylinder giving greater hydraulic force.

[phil13]

Hi, olds.
            Yes the cylinder is for rear brakes only, be it drums for now and later discs. I'd rather just replace it just the once, so hopefully can find one that would fulfil both tasks. The old m/c is a girling(guessing because it has a  G stamped on the body)  on the side between mounting holes it has a 3 and a J cast into the body and between those are etched/engraved 412+. If the original has the humph to operate the reliant drums and that will also supply enough humph to do the discs then that may be the way just for ease of fitting.
Many thanks phil ...

[kapri]

Movement on the disc is less but diameter of piston is much larger (Pi x r2 xH) .More volume is required to move discs but larger bore m/c also produces less line pressure.  As we are only talking one m/c powering half a normal system the change in pedal travel will be hardly noticeable . However using a drum m/c on the discs will result in stronger braking force to the rear than using a disc/drum set up m/c, albeit slightly longer travel with the smaller m/c  . 5/8 for all drum, 3/4 for drum disc set up.

[minimutly]

I reckon you'll find you'll be pushing that pedal a lot harder than with drums, but I would go with the mc you have until you try it.
Discs work well in terms of fading, snatching, etc, but there's a reason they all have servos.
Let us know eh?
Huw

[Olds]

Your existing master cylinder probably just needs new seals. A pic might help in identification. Any particular reason for changing to discs ?

[kapri]

I reckon you'll find you'll be pushing that pedal a lot harder than with drums, but I would go with the mc you have until you try it.
Discs work well in terms of fading, snatching, etc, but there's a reason they all have servos.
Let us know eh?
Huw

Indeed, the reason is that they normally need a larger bore m/c to supply enough volume but the bigger bore means at reduced line pressure hence servo on  4 wheel system. However racing systems use tandem m/cs ( 2 side by side ) so you get the volume AND pressure without need for servo . They feel strange to start with , especially when you are used to servo brakes but work great. On a trike with a separate rear system you are effectively using that system.

[phil13]

Thanks for all the replys ,  apologies for being nieve ,the G  stamped on the cylinder body that I thought was for girling infact is brembo 's motif. Why it lookalike a G I don't know?  So it is a brembo master cylinder, I will check up later for the serial no , and price a replacement.  I'm changing to disc because    1. I have most parts. 2. I like a challenge. And 3.  My rear brakes have never been good ( foot or hand )  and fancy a change.
Cheers Phil .

[BikerGran]

They may have been not good because the M/C was failing?  I have the original Reliant drums on my trike and they're plenty powerful enough!