75 Commando Front Brake woes

About at wits end. Ok here goes. 75 Commando MK3. Calipers were draggy, pistons froze, so sent off both front and rear calipers to Classic Bike Experience in Vermont for rebuild. Re installed and then during bleeding noticed some fluid around the front brake lever. Removed the Master cylinder to see that it had been sleeved to 13 mm. Ordered a rebuild kit and re installed the Master. Bleeding again, now discover a blow out on a front hose, so measure and order a one piece Stainless line. Again bleeding , with all the tricks to get a decent lever. Could almost pull the lever to the bar. Tied the lever back with bike on side stand with Master as high as possible. Next morning, very good hard lever,,, Success!! nope not so much, within several hours, lever is back close to the bar? Put a new speed bleeder in the front caliper to make sure it was sealing. I can't find any fluid leaking so how is air getting back in the system ? If I tie the lever back over night it produces a good hard lever in the morning, but within hours its soft.
Looking for any ideas, before I plunk down $250 bucks for a new master cylinder to eliminate one more possibility.

I followed similar path as you ... never bothered with original m/c though .... got the Brembo m/c kit from CNW and great front brake ever since ... good luck , I can’t see why your brake is still spongy , sorry

Sounds like there is still air in the system. Try bleeding slooooooowly. Try using a syringe and add fluid at the bleeder. Try gentle rapping on the caliper to dislodge trapped air bubbles. Try removing the caliper (hose still attached) and rotate it around & try to get any trapped air to the bleeder and bleed again, sloooooowly.

I have bled traditionally , vacuum bled, and reverse bled with syringe. I have tapped etc. The mystery is why would I have an outstanding hard lever after tying the lever back and letting it sit overnight, then a few hours later be able to pull the lever 1/2" to the bar???????

Just using logic,... it sounds like somehow your master cylinder's bypass port isn't functioning properly. When you release the brake lever, the piston cup retracts because of the spring in front of it. The cup clears the bypass port which is suposed to release the hydraulic pressure on the line so all the braking pressure is gone. The pad and piston normally only move away from the disc minimally since there isn't any negative line pressure to pull them back very far because the bypass port functions to release all line pressure.

Instead, with the bypass port not functioning properly, the spring on the master cylinder's piston is actually drawing a vacuum on your line when you release the lever, drawing fluid pressure back out of your caliper pulling the calipers pistons (and maybe the brake pads with them) back away from the disk. So, after you use the lever a few times, a few hours later the pads and disc have been slowly drawn backward by the hydraulic pressure that remained because of the disfunctional bypass port. The caliper's piston and brake pads then have to travel a long distance to contact the disc, which is percieved and a long lever pull to get any pressure to the disk...

*and now my opinion...which is a penalty you pay for my attempt at an answer... (yeah, I'm joking) I had a lockheed master on my commando for most of the 40 years I've owned it. Every 5 years or so it would leak and ruin the paint on yet another gas tank. I would buy the rebuild kit again, rebuild the master, repaint the tank and go another 5 years until it leaked again on the tank and do it all again... This last time I was sick of that, so I bought one of Don Pender's (madass here in the forum) aftermarket master cylinders which has the perches to mount the standard commando switch clusters on them. I think it's a 13mm sized piston. It was very affordable and came with a braided line. It has a great feel to it, and doesn't leak. Personally, I'm not much for keeping things stock for stock's sake or for future value, but you could always save your stock master cylinder for that reason if you replace it with Don's. I sold mine here and recouped half the money I spent to by Don's master which I am much happier with... HTH

Your theory sounds plausible, and better than any I can come up with. I did suspect the Master might be the cause, just weird that before the caliper rebuild it was fine? Will the Don Penders Master work on a 75 MK3, the switch cluster is unique I think? Also the one piece stainless brake hose I have screws directly to the factory Master,,,, wonder if the threads are the same on Don's??

I have fought this problem twice on a MK3. My only solution was to make a spacer from 1.4" aluminum plate and put it between the MC and the switch box. The lever position is reasonable and the brake works well. Another way to solve the problem is to fit a lever from an earlier year but now there is a very long reach for your fingers. The problem is trying to push the same amount of fluid through a smaller chamber...it takes more lever travel to do it. If I am faced with this problem again I will cut a spacer and save months of headaches.

here's the link to the master I bought from him. His replacement master cylinder is actually 1/2" diameter master, not the 13mm that I stated previously.

http://www.tritonmotorcycleparts.com/297620485/category/146825/online-store

He also makes a resleeve kit for the stock lockheed master, but I had enough of that master's failures as I stated in my last post, so I went for the modern ungrade since my bike is a frankensteins monster anyway. The new master is on that page too with various other stuff he makes.

It's the piston seals, they need time to settle. Also, unless you can remove the trapped air in the MK3 caliper it will always be spongy, vacumn bleed and mitivac etc are not always that good.
The caliper was designed for the pre-MK3 and when you look at you will see why those with MK3 find it harder to bleed than pre mk3. You will also note that the bleed channel is also the fill the channel to the inner bore, which is not ideal.
Once you have 'all' the air out take bike off centre stand with reservoir cap fitted and lean bike slightly to one side and rock back and forth, lean other way and repeat, this will allow the air that sits in a top pocket in either bore to travel to the bleed point, re-bleed job done. The other way is to remove the mudguard bridge stay and use a piece of batten wider than the disc and manipulate caliper with bracket to aid air escape.
Once you have a brake you can use, ride for a few days and re-bleed, the lever feel should improve further.

Once you try a Brembo or Madass conversion......I have one of each on 2 bikes you will realize just how bad a standard Norton brake is, no matter what you do to it, and I have tried most things...they were far from good when they were new!....riding one with original brakes now is scary.......they should be made to stop faster first.... before wasting money on going faster...IMHO

As Madnorton suggests, remove the caliper from it's bracket with the brake line still attached and manipulate the caliper using gravity to make any trapped air in the caliper to go toward the bleeder, then bleed it again. It's smart to try this first, before you conclude it's a master cylinder issue. Manipulating the caliper only costs you some of your time to make sure you've eliminated the possibility of air trapped inside the caliper. The new master cylinder cost money and assumes you are replacing the part at fault. Certainly, my diagnosis is only a guess from your initial discription here, so it would be prudent to eliminate air trapped in the caliper as a cause first, before you buy a new master.

I have bled my share of Norton clam shell calipers/OE master systems; can be quite frustrating, as you well know. The Madass or CNW front brake offerings are as modern and effective as you can get without ABS; and I'm not a fan of ABS systems.

Your technique of tying the master's lever back seems counter intuitive to me as you are functionally sealing the fluid/air from returning to the master, very much like a line lock. I'd suggest that after going through your bleeding ritual that you insure that the caliper fluid supply line hole is the highest point, that you then hang the master (untied) so that the reservoir is the highest point, and be sure to cover any paint work that the master may be hanging over. And do check to see if the lever isn't pressing on the master's piston, this can close off the return/feed holes on the masters body.

My theory is that one or both of the square "O" rings that seal the caliper pistons may have flipped (upon assembly) or the groove that it (they) run in may have a ridge of corrosion that is accentuating the retraction function of the "O" rings. Setting the caliper pistons in place is difficult enough.

When I rebuild a clamshell the last effort is to put a piece of plywood between the pistons and hit it with 60 psi compressed air. Both pistons should come flying out, ideally the same distance, and grab the plywood with a death grip. When you remove the air pressure the piece of plywood should be easy to withdraw. And the pistons should withdraw by not a lot more than .010" on both sides. I run this routine several times to give the pistons and seals time to get to know each other. I leave enough brake fluid behind the pistons to provide a modicum of lubrication for this exercise, I also push the pistons back in their bores to judge the feel of how that retract. If the seals are going to leak you'll see it or hear it.

In short summary:

The caliper pistons are retracting too much, or the hydraulic system does not have clear feed/return channel.

Best wishes.

Well the madass Master Cylinder is not an option as it will not work on a 75 Mk3 due to the different switch package.
So I will go thru several of the suggestions made here and see if I can find any air hiding in the system, before plunking down $300 for a new OE Master.
By the way did I mention, this is frustrating !!!!

Do Andover Norton supply a MK3 mastercylinder with a 1/2" bore? If not then it would be a pointless exercise spending $300 just to get back to square one with a standard wooden brake. I dont know how many resleeve kits I've sold to MK3 owners.

MK3 Master cylinder 13mm bore

https://andover-norton.co.uk/en/shop-details/18714

This one has already been resleeved. Yes, Steadfast Cycles sells a new 13mm MC for 75 MK3's.

AN used to make a Grimeca bolt on for MK3 that worked with the original switches, but do not supply it now, I have one on mine, works great......

Rocky Point Cycle sells the OE type Mk3 masters that have been sleeved down, for considerably less than $300.

It's air in the MC. Someone, old Brits perhaps, shows a picture of the MC removed from the bars and held vertically whilst pushing fluid up from the bottom. It's the only thing that has worked for me.

My theory on why tying the lever back works temporarily is that the pressure forces the trapped air into solution with the brake fluid thus becoming non compressible. Shortly after the pressure is released the air comes back out into a compressible gas and the sponginess returns.

I have a AP caliper on a 12" RGM disc with a sleeved MC. It's still a bitch to bleed but the best brake I've ever had.

Get a 20 or 40cc syringe and an appropriately sized tube. Home depot has the tubing for replacement of small engine fuel lines in various sizes.

Treat the brake fluid like it was nitro, don't shake it, slowly draw it in with the syringe and then check for any bubbles. Squirt some out so the column is clear.

Remove the MC. Dump it out and replace the cap. Hold vertically and move as allowed by the hose whilst filling through the bleeder from the bottom. Don't let any new air get in. Repeat as necessary.

Good luck, we're all counting on you.

Edit: I've been surprised to find bubbles inside the syringe even when carefully drawing in fluid. Tap the syringe and get every last bubble out or you'll be frustrated.

Another Edit:

Reflecting on the behavior of air bubbles in the syringe I think the above procedure could be further enhanced by enlisting a helper to hold the master cylinder vertical moving it in an orbit AND tapping it with a screwdriver handle simultaneously. I've always been alone during the process but next time will have help.