Front brake..primary cup?

OK....now it is time to learn about brakes. My front brake is now doing the deal where it works fine but with steady pressure on the lever, it will pull all the way into the grip. No fluid leaking out of the system. Endless hours bleeding and re-bleeding until I think I have all the air out. I have read the other posts about this and one answer was that it MIGHT be the primary cup in the master cylinder. My question is, what eles should I check before tearing into my master cylinder? If the answere is nothing else to check then in I go. Can't wait!

Do you still have the stock rubber lines? If so, change to some braided-stainless lines to get the feel back.

A bad oil ring or scored bore may bleed off pressure but not show as a leak.
SS braid hose does help transmit more your squeeze to the pads.

If you find it is scored in the bore, you might want to opt for the 13mm sleeve modification. The only other option is to get a new master cylinder. Suspect the modification is cheaper.

the raw steel master cylinder pistons rust from condensation, this swells and can un seal o-rings or create pores fluid can pass pressure off. Same at calipers.
Given time the Al bores gets pitted. If so, Resleeve is way to go with SS piston and pucks.

Hope it don't apply to you, but I'd had hidden leak take brake away completely from one grab to the next - gone. Check level at each gas stop to be sure it staying up.

Commando 75,

If I read u right, the lever keeps pulling in toward the grip? That is a damaged cup seal, leaking internally. So it will not show up as a loss of fluid. If you release and re-apply you will get another pul, that will slowly subside.
When you pull the MC apaetlook for a small nick in the cup, maybe in the shape of the small breather hole back into the reservoir. This indicates the piston cups are not correctly orientate to the breather hole porting. I know because I made my own smaller diameter piston, and it worked fine until I had your problem. Also from my experience in making my own pistons, if the relationship with the piston to MC is not correct, I.E. when you pull the lever it does not fully stroke the cyclinder bore, or short strokes, it wil make bleeding a pain and you will end up with a spongy lever. I added a button to my piston, between the piston & lever and it bleed up like magic.

Good luck

Richard

Once again thanks so very much for all the info. I took the MC apart today just to get it over with. As usual...nothing scary in there. Richard thanks for the ID of the cup seal as the problem. It looked OK to me but I will tear it apart tomorow and really check it over. Also Richard what do you mean 'added a button'? A spacer of some kind?

I can see where the brake hoses could cause the brake to feel odd but I am pretty sure the symptom of the lever pulling all the way in is a master cylimder issue. It feels exactly like under the constant pressure fluid is escaping to the other side of a seal.

While it's apart it's a good time to have a stainless 12mm or 1/2 inch conversion done and braides hose. What a difference !

graeme

Commando, I am runnign a 1/2" bore already, are u running std 5/8"?
I made up a new piston to some dimendions that were posted by a local Norotn Commando Owners club. And I made the piston from alloy with a bronze end button, for the lever to strike on. With the NO dimensions in reference to lenght and where the seals are placed, I tried bleeding up, but could not get a full firm bleed. So I popped the piston out and made a 3mm longer bronze button, these are pressed into the end of the alloy piston assy, interfernece fit. And lo and behold after pushing the piston back in the damn thing had excellent feel withot even re-bleeding.
So if you have an aftermarket piston set up, it could have the same issues my piston had. But if the seal has a mark in it like a little round hole, that indicates it is too far out of the cylinder when pressure is building, and that causes the seal edge to get extruded out through the reservoir hole,

Hope this helps,

Richard

Butt in...
I'm in a similar situation with my MK111,
Not meaning to hijack, where would one purchase only the rubber cups for this 1/2 or any other conversion.
I know where to get the whole stock Norton and the re-sleeve 13mm kit, I just want a variety of the cups.
Can they be bought from a catalogue maybe?
Thanks.
Butt out...