Master cylinder issue

Hi all
I've just bought a 1974 Mk 2 and am having problems with the front disc brake. Sometimes it works and other times the lever goes back to the bars with no effect. There are no fluid leaks and there were no bubbles when I bled the brake. According to the description (I bought at auction) the brakes have been overhauled and the hose is a new stainless steel one. I believe the same guy overhauled the electrics and I've already fixed some stupid mistakes there.
Not being an expert on disc brakes I suspect the master cylinder is faulty (seals working intermittently?). I don't trust myself to rebuild it and am pretty sure it is a pattern one so I'm not sure what rebuild kit to buy anyway so I'm thinking of replacing the whole master cylinder. Now to my question (at last!), I can get a master cylinder assembly for a 1970s Triumph for less than half the price the Norton merchants charge - is there any reason why I shouldn't fit this given many people seem to fit alternative master cylinders as an upgrade. This looks like it would just bolt on as Triumph and Norton used the same Lucas switch gear. Any thoughts, advice or warnings appreciated.
cheers
Pev

Quite possible the person who rebuilt it put the seals in wrong. It's a slightly complicated set up, as to fit the piston seal it has to go over one lip, then slide along a section and then go over another lip. I've seen them fitted in the wrong place, and even a new one slid onto the first section and then old one still sitting in the correct place.

Having said that it's not that difficult to get right either. Why not just research the proper assembly order (look in the workshop manual, or no doubt avilable online) and do it yourself, or take to your local friendly bike mechanic, if only new seals are required no one should charge more than say 20 quid to fit them.

Regards,

Clanger.

pev,

If you are going to replace the master cylinder do not go with the stock ⅝" piston. A smaller piston diameter, 12mm-13mm will give you a much, much better front disk brake than a stock ⅝" diameter piston. Do a search on this site and you'll find a ton of info. I highly recommend you view this site:

http://www.vintagebrake.com/mastercylinder.htm

BTW, I ran a 11mm master with 42mm, 2 piston caliper and it's one sweet brake!

JimC said:

pev,

If you are going to replace the master cylinder do not go with the stock ⅝" piston. A smaller piston diameter, 12mm-13mm will give you a much, much better front disk brake than a stock ⅝" diameter piston. Do a search on this site and you'll find a ton of info. I highly recommend you view this site:

http://www.vintagebrake.com/mastercylinder.htm

BTW, I ran a 11mm master with 42mm, 2 piston caliper and it's one sweet brake!

Click to expand...

Hi Jim,

The down side to the smaller ID conversion is it makes the master cylinder a pain to bleed. Maybe what is ailing in this case.

Hi Jim,

The down side to the smaller ID conversion is it makes the master cylinder a pain to bleed. Maybe what is ailing in this case.

Click to expand...

Never heard that one before. Why would a smaller piston be more difficult? Possibly because less fluid is moved per stroke if you are using the master to bleed the system? I power bleed all brake systems and seems to have eliminated any brake bleeding hassles. Regardless, more difficulty bleeding is a small price to pay for a much improved brake with the correct caliper to master ratio.