MK# front master cylinder, fine details please.

seattle##gs said:

What is the function of the primary cup washer?

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seattle##gs said:

the "primary cup washer" has a convex side and a concave side..sort of. I assume the convex side faces the primary cup and the concave side faces the steel plunger. Am I right?

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Factory manual, Section H:

Well, I installed the washer and I am guessing it was the wrong way as I could not get any real improvement to the brakes. It doesn't look like it would make much difference but SOMETHING isn't working. Also, the retaining circlip does a poor job of holding the contents in. As I was removing the lever the innards popped out and began draining the reservoir onto the ground. As it was 3 am I threw in the towel for the eve. The only thing I haven't done is replace the rubber hose which appears to be circa 1975 but passes fluid very well. It is fairly thin wall by modern standards and replaces all the metal lines except the one on the caliper.
I watched the two brake pads move in and out. The outboard pad clearly did what it was supposed to do...it would move forward and then spring back a whisker when I released the lever. The inboard pad made no such movement. It stayed against the disc the whole time.

Is it possible that your 42-year-old rubber hose is acting like a balloon, robbing some or most of the pressure needed at the pistons?
I noticed a marked improvement on my brake, with the installation of a modern line.

seattle##gs said:

The outboard pad clearly did what it was supposed to do...it would move forward and then spring back a whisker when I released the lever. The inboard pad made no such movement. It stayed against the disc the whole time.

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That seems to indicate what I said to you on the last page. From that description it sounds like it could be the caliper, not the master cylinder that is the problem. On some cars, the whole caliper floats and only one side of it has a hydraulic slave cylinder. You lube the "pins" that the caliper floats on so the non hydraulic side of the caliper moves away from the disk continues to wear BOTH pads evenly while the hydraulic side releases it's pressure on the slave cylinder.

ON a dual piston caliper like a commando, BOTH brake pads have to move while the caliper stays put. If this isn't happening, it sounds like the caliper to me, not the master cylinder.

I put new seals in the caliper after cleaning the passageway from outboard to inboard, there was not a spec of crud. I cleaned the caliper and the seal grooves better than I ever have and reinstalled the stainless pistons that were in excellent condition. On the bench without seals the piston slid back and forth easily on both sides. I lubed everything with brake fluid and pushed the inboard piston in just enough to clear the disc. That should allow the fluid to get in behind the inboard piston. However, maybe it didn't. I could try removing the caliper while keeping it connected to the master cylinder, put in a piece of 3/16 aluminum plate and and apply compressed air through the bleeder port and have the inboard piston move a tad forward, perhaps allowing fluid to fill the inboard side. There isn't much to go wrong with a caliper, it is a block of aluminum with a couple of holes drilled so the fluid can go back and forth.
It is also possible that the hose is the problem, I need to take it to the local brake supply house and have them make up a custom line.

The front caliper is mounted in a way that the bleeder screw is at the highest point so it shouldn't need manipulation to bleed it all out...correct?

Considering that air is compressible and is the element we are trying to purge, I don't think I'd want to apply it intentionally into the system.
Yes, the bleeder screw is the highest point of the caliper, but bubbles can be trapped anywhere up the line also, and even in the MC. It wd be interesting to see a cutaway view of the caliper, showing the paths of the passages; maybe there are internal "high spots" where bubbles can hide, stuck to the walls?
I think that's why a lot of people do the "reverse bleed", injecting fluid into the bleeder valve and pushing the fluid with any bubbles up, so they can escape to atmosphere in the reservoir, which is the direction they try to move anyway.
Almost everyone recommends tying the lever back to the grip after bleeding, and leaving it there overnight, before looking for other problems.
I'm following this with interest, as I will soon be installing Don Pender's sleeve kit for the MC.

I am starting to rebuild my MK3 front brake. I will take the time to photograph the parts, process , and write a description. Probably start another thread.

Any news?

After a frustrating week I sent it off to Fair Spares and it seems the MC needs a replacement so, $250 later, perhaps the brake will work well. will let you know if that was the problem all along. His eyes must be better than mine, I could see nothing the matter, but then again, I got nowhere with the usual methods.

Are you getting the new one with smaller bore?
https://andover-norton.co.uk/en/sho...body-c-w-mirror-mount-13mm-bore-c-w-internals

You might look over my thread to see if you or I missed something. My brakes will need bedding in, running the bike up and down the street for a 1/2 a mile was not enough, although the front started feeling better even then, it is winter here

https://www.accessnorton.com/Norton...linder-and-caliper-rebuild-with-photos.24231/

Mr. Rick said:

Are you getting the new one with smaller bore?

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good idea , the 13mm one without a sleeve from AN

jimbo said:

good idea , the 13mm one without a sleeve from AN

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And which does not seem to include the valve assy 06 1944, recommended for the hot nail treatment by Hobot...
Too many variables!