Rear master cylinder rebuild

Previously wrote:

I've just rebuilt the rear master cylinder on my 1975 MKIII and fitted a Grimeca caliper and stainless hose. I'm sure I've put all of the seals in the right way but when I put brake fluid in the reservoir it just stays there. I've pumped the pedal and still nothing. I can hear air coming out of the caliper bleed nipple when I press the pedal. Do I need to prime the master cylinder somehow?

I can't find an exploded diagram anywhere for the rear master cylinder to double check against.

I don't know if I can face taking it all off again.

Any ideas?

It's sunny and I should be out riding and not trapped in my garage.

Totally fed up Shaun from Headcorn!

You could try reverse bleeding, this involves pushing brake fluid into the caliper nipple with a syringe and a length of tube, back to the resevoir, then bleed as normal, sometimes its difficult to get all of the air out of the caliper thenormal way,I used this method on my kids motorcross bikes and it always worked. Hope it works for you,I am just going for a ride myself, (Big 4, Drum brakes)! Cheers, Ian.

Previously wrote:

.......I don't know if I can face taking it all off again.

Any ideas?

It's sunny and I should be out riding and not trapped in my garage.

Totally fed up Shaun from Headcorn!

Stick with it Shaun.

I hope you have someone helping you; because another pair of hands are a plus with this.

Not knowing whether you have bled an hydraulic system before, youâll have to put up with some advice which may be âstating the obviousâ.

I hope you have some reasonable basic equipment for the job.

A decent length of clear pipe, with good, rubber ends is the best.

There are a lot of fancy âHalfordsâ type kits, with one-way valves; but the right pipe, a good ring spanner for the bleed nipple, a jam jar for a catch-tank and lots of rags handy for spills (& an assistant!) are the essentials.

Oh, and some good, fresh brake fluid of course.

Have the catch tank on a secure surface, about level with the bleed nipple. Balancing a jar ? full of brake fluid usually ends in tears!

You wonât start to âget a pedalâ until the fluid reaches the caliper.

Are you nipping up the bleed nipple once the pedal is fully (and held) down?

If you arenât doing that, air will be drawn back into the bleed pipe as the piston is returned in the m/cyl when the pedal is released.

It always takes a while for the fluid to reach the caliper when bleeding. There ought not to be any need to prime the caliper; but the air in the system will be compressing, then expanding while you pump the fluid through.

Tightening (not too much!) the bleed nipple when the pedal is fully down, then loosen only when the pedal is being pressed again should bring the fluid through; but get someone to give you a hand.

Thatâs all Iâll offer for now.

Let us know how you are getting on.

Paul

Hurrah! Thanks Guys. I did consider Ian's idea but I lacked a handy syringe! You were right Paul, all I was doing was pushing air along the pipe and when I released the pedal it drew the air back again. My brother helped me and it was a case of open bleed nipple, press pedal, close bleed nipple and so on. That way the air was finally pushed out and it drew the brake fluid in.

It seems obvious to me now but but sometimes on these projects the simplest things are the most evasive!

I have now to do the front end with large disk, Grimeca caliper and, if I can work it out, a Landsdowne fork conversion kit. Cost me a fortune but apparently transforms the handling considerably. We shall see...

cheers

Shaun

Previously wrote:

Hurrah! Thanks Guys........................

...........I have now to do the front end with large disk, Grimeca caliper and, if I can work it out, a Landsdowne fork conversion kit. Cost me a fortune but apparently transforms the handling considerably. We shall see...

cheers

Shaun

Thatâs a result.

Well done! The caliper looks like a nice piece of kit.

Ianâs suggestion is a good one, which Iâve used myself a few times. Luckily my family used to farm, so a scary looking old syringe now lurks in my shed for those special occasions.

Iâm curious about that Landsdowne conversion for the roadholder forks. Is there a link to a website, with some more information?

I havenât done any mods on my Dommies; but having exactly 0.005â clearance between the damper rods and the little caps they pass through, on top of the damper tubes, does give good damping. The diameter of the damper rods varies quite a lot from new; so spending some time with a micrometer, while the parts man rolls his eyes as you select some good ones from his stock, is time well spent.

Reproduction stanchions come in various diameters as well as sometimes being banana shaped. So how people can buy them on fleaBay beggars belief.

Fitting slightly longer damper rods and using small lengths of spring to space the resulting gap above the fork springs is supposed to allow the dampers to work properly.

Then you can get involved with moving the holes at the bottom of the dampers around and fitting longer top bushes to give proper hydraulic stops.

Increasing the amount of fork oil in Commandos to 200cc is recommended.

Gerry Bristow noticed a big improvement after changing the fork oil to 15W/50 Mobil1 on his Mk111.

Always something to do on a Norton, apart from riding it, ay.

Paul

Hi Paul

Try this link. They cost me £215 including postage. They look very well made and allow you to adjust the fork damping.

http://www.accessnorton.com/lansdowne-fork-damper-kit-t4683.html

Below is an email from John Bould who makes the kit and also his reply when I asked if I could use RGM's progressive fork springs which I had just bought:

Hi, Shaun,The kits being a "Dial in" adjustment to control the excessive travel of the original system. Original units are replaced by a "TOTAL CONTROL". This system via top mounted adjusters allows you to "Dial in the ride" you want. Designed as a simple drop in replacement..requiring no special tools or machining...refit is normaly 2 hours..Kits cost £215 post free...Paypal is jrb19502005@yahoo.co.uk dispatch is normaly 7-8 working days.

Progessive springs where used in Nortons to combat the excessive dive on braking. The lansdowne kit as built in "anti dive"...the dampers have compressional resistanceYour new springs are OK. Points to bare in mind...1 ensure correct size bush's are employed...the old bronze are better than the brass ones, 2 check the wheel spindle hole is good...no excessive slack.3, the yolkes are not twisted...common! stanchion chrome is good and renew the seals...wash out the legs ...Regards John

Hope this is of interest.

Regards, Shaun

Thanks Shaun,

John Bould raised a relevant point in his message about checking the wheel spindle hole in the timing side leg.

I wonder how many owners have measured that...?

I hope the job goes well.

I recommend that while you have the front end apart, you find an engineering shop who will measure your stanchions for diameter at various points and for straightness.

Once you have it done and shaken down a bit, an article in Roadholder would make an interesting read, especially with some good pictures.

A new thread on this site would be worthwhile, that could air some views on various ways to set up roadholder forks.

Youâre obviously dead keen to make your Commando a pleasure to ride.

Good on you!

Paul

The Lansdowne kit for Roadholders as been tested by John Dunn the Tec Adviser,i am am sure he will be happy to answer any questions regarding the Kit., available for all roadholder types from early long [pre 1953] to later Commando...Best wish's John Lansdowne Eng Email jrb19502005@yahoo.co.uk or call 01782519145