New front master cylinder

[Wired1]

I have changed the front master cylinder on my SR400 to one that has a brake switch as I need to have a brake switch to get a warrant. I had another front master cylinder in the shed that came off another project bike several years ago. It is made in West Germany but other than that I can't determine the make and it certainly isn't standard equipment on the XV750 I took it off so no idea where it came from.
Anyway, onto the problem. The new master cylinder installed and bled doesn't seem to want to apply enough pressure to the front pads. Now the front pads have had fork fluid over them so I have take the precaution of ordering a new set which should be here in the next day or so. But even with these pads the brakes did work before with the old master cylinder.
So I'm wondering if the new master cylinder doesn't have enough volume of push or however you measure the amount of fluid moved, or if it is buggered. Or maybe it's been sitting too long and has deteriorated some way. It locks the front up on the second pump so I'm thinking it needs more bleeding. But I've bled the reservoir empty three times and it's still the same.
Any wisdom would be appreciated, I don't want to go to the brake shop but that's the next step if I can't find a silly mistake :)

[Squiggles]

what do the seals look like around where the lever meets piston, sounds like you're getting air into the system, thus the "priming pull" effect you're experiencing as the air moves out of the system/is compressed on the first time round, and the fluid actually moved the second

[Kickaha]

Is it the master cylinder the same diameter as the old one?

[Wired1]

I guess I'm asking the question because I suspect the new one isn't big enough. I haven't had either apart but I suspect that the replacement unit has a smaller piston diameter and therefore pushes less fluid. I will check the seals today and pull it apart to see what's going on inside it. My front wheel is away being powder coated and wrapped in street legal tyres this week so I can't try it out on the disk for a few days.
I have been looking at replacement master cylinders on ebay and they look pretty affordable for new ones, how do you size them?

[imdying]

Sized on piston diameter for the most part, to a lesser degree, stroke length, although that's generally of a neglibile difference. The size will be cast into the side of it.

[Wired1]

Sized on piston diameter for the most part, to a lesser degree, stroke length, although that's generally of a neglibile difference. The size will be cast into the side of it.

That's probably the info - the one I took off says 14 and the one I put on says 11 so that's 11mm diameter to 14mm diameter. The two units I was looking at on ebay were 12.7mm which they recommend for the XT600 (so a single disk bike of the same size) and 14mm for the XV750 (twin disk bike slightly heavier).
Is there a down-side to buying the 14mm one for a single disk?

[imdying]

It's not as simple as that, as it depends on the size of the caliper piston.

Too small, too much lever travel is requied.
Too large, and the lever goes rock solid, but gives no feel, much like fitting braided brake lines in some application. Some people love that feel (or at least think they do), some don't.

On a sportsbike, ideally you should be able to two finger brake on minimal travel, and still have good feel... which is a bit of a challenge, but generally the factory gets it pretty right. Often lines are replaced with braided giving a much firmer lever which is because the 'shitty rubber lines are now gone'... when in fact it's a much firmer lever because the 'worn out rubber lines are now gone'.

That's a little off track though. On your custom styled bike, I would be looking to fit a master that gives good feel over anything else, as max effort last ditch braking is probably not a typical situation, so more value would be garnered from better feel.

So, given all that dribble... how did the old 14mm master cylinder feel in your application 6 months ago? (i.e. before it was giving any trouble). There's no reason that we couldn't just rebuild that master cylinder if it:
a) performs as desired
b) has the correct fittings/threads
c) has a functional brake light switch

Worst case, the bore is corroded and unserviceable, which means resleeving it with stainless steel and resealing the piston... typically, on something like that, around the $100 + freight and gst cost.

But, facts first, I'm still a little confused as to your application.

You have an SR400 that has functioned fine up till now, with a 14mm master cylinder, and a single disc front end, but doesn't have provision for a brake light switch no?

[Wired1]

Hi, yes more or less. I have an SR400 that had a functioning front brake but no brake switch. I need to get a warrant to be able to register the bike and there are some difficulties with the history of the bike (a long story) but basically I have LTSA happy with the history for now so I want to register it as quickly as possible before it all changes again. Phew. So I had a spare master cylinder in the shed (the 11mm jobbie) that has a brake light switch so I figured it was a straight forward job to swap the master cylinder and wire up the brake light switch.
Well the electrical bit went fine but not so the hydraulic. I have bled it - maybe not enough? - and it now works poorly with the new master cylinder (the brake light works perfectly!). The disk locks up on the second pump.

I don't know anything about either master cylinder, and I can't be sure the 11mm one is a goody.

[imdying]

It appears from your description that the 11mm moves an inadequate amount of fluid for your application, these are your options.

1) Get a banjo bolt (OF THE SAME THREAD!! :D) with a pressure switch built in.

I am not a fan of these. They require pressure to activate. A conventional mechanical switch does not. This means that the brakes need to come on before the light can come on, not the other way round as it typical for every vehicle on the road. Plus, they have a shorter service life than mechanical switches IME. They're acceptable for rear brakes when fitting race style rear sets that preclude the fitting of the factory rear brake light switch, but not desirable for the front. This is why Brembo makes their top end master cylinders in two versions, one with a switch, one without. Having said that, they do work, and would solve your problems.

2) Get a 14mm master cylinder (with a switch) off just about anything, and fit that. There's heaps of them about, go to a wrecker and grab 3 or 4, and bring him back the ones that didn't work or fit or whatever.

3) Buy a brand new 14mm master from eGay.

4) Contact someone like Cycletreads and see if they have a generic 14mm master with a switch that you can have shipped to your door via the wonder of credit cards and couriers.

[Wired1]

I dug out the old-old master cylinder today. This bike was in a shed that caught fire and a lot of the front end was melted including the speedo, tacho, wiring loom and master cylinder. The master cylinder that was on this bike when I got it was a stand-in for the melted bugger. Anyway, the one on the box of melted parts that came with the bike is indeed a 14mm so that settles it I suppose - I need a 14mm master cylinder.

The melted one had a plastic reservoir which is a lump of plastic slag, but the body is still in tact missing the lever. I squirted some crc down it and gave the piston a light tap with a pin punch and it does move but is sticky. To fix this original master cylider I would need a lever and a reservoir plus probably a master cylinder kit whereas I can buy a master cylinder off fleabay for $80 US including shipping brand new. Is there any point fixing this old one? can I even get the parts? I think the answer is obvious.

[vagrant]

Does your rear brake have a switch to activate the brake light?

WOF regs http://www.ltsa.govt.nz/certifiers/virm-in-service/motorcycles-04-v3.pdf

section 4-10 para 9 in the grey section says "a stop lamp must come on when "A" service brake is applied". It doesn't specify which brake system.

It's not the best way to do things, but if you are in a hurry to get it WOF'd and reg'd, then it is passable.

You will have to remember to do all your braking with both ends.

I would not suggest this as a "normal" ridable solution, but something to get your admin done while you source another front master cylinder.

Good luck with the rest of your project.

[Wired1]

Thanks for that. I asked the VTNZ guys for a list of what they check and the fogged me off with some bullshit about it being "pretty obvious stuff".

http://www.landtransport.govt.nz/certifiers/virm-in-service/motorcycles-v3-2.pdf

Reading through this list I can't see speedo anywhere - can it be that you don't need one for a warrant? OK forget that I found it in the larger document. I see I also need to provide the pillion with foot rests - bugger.

[imdying]

I can buy a master cylinder off fleabay for $80 US including shipping brand new. Is there any point fixing this old one? can I even get the parts? I think the answer is obvious.I've yet to see one that can't be fixed, although the reservoir would be a pain. For $80US, there's no contest in my eyes.

[FROSTY]

Seems to me you might be looking at this problem backwards.
You have a 14mm master that works the brakes just fine.
So why don't ya mount a brake light switch on this one?
if all else fails a simple microswitch will do it

To mmaybee help you can ya take a picture of the underside/front of the 14mm unit

[Wired1]

Yeah good call. In retrospect I should have tackled the switch problem rather than swapping in a completely untried unit. The original master cylinder is fucken ugly but did the job so when I get it all back together this weekend I'll probably go back to that master cylinder and bungy a micro switch on the side. It does have a space for one but it is such a crappy looking unit that I thought it was easier to ditch the whole lot and put the other tidy one on.

I have had the wheels into the powdercoater this week and then in for new tyres so while they've been away I replaced the fork seals and got the forks all tidied up. My speedo turned up today as well so that was the last thing I needed for a WOF so I'll put it all back together tomorrow and hopefully get it in for a warrant later next week. With a warrant I can revive the expired plates and then it's legitimate again. Then I'll pull it all apart and do a proper restoration on it...