rear master cylinder fails bench bleed

[steve112112]

'78 XS1100, rebuilt the rear master cylinder as part of a restore on a bike that has been sitting for 10+ years. Everything else is done on the bike, carbs sync'ed, petcock's replaced, front master cylinder replace. The bike runs great but can't get the rear brake working. Rebuilt the rear MC but having trouble getting it to pass the bench bleed as described below. With my finger over the brake hose, on the first pump I feel pressure and can let air esacpe. Then I cover the line again (with brake pedal still held down) and repump but don't get any pressure. If feels as if it is not getting any fluid through even though I took it completely apart and all the holes (even the spoonge) seem clear as I blew compressed air everywhere I could. Any suggestions what to do next? Should I replace the MC? If so, any recommendations as to which one?

Suggestions very much appreciated,
Steve

You DO need to "bench bleed" the master! If you don't, you will never get it to work correctly. With the brake hose off the master, put a finger over it while you depress the piston. Let the air escape, but seal with your finger BEFORE letting off the piston. Repeat until pressure and fluid come out, then hook everything back up and bleed the system.

[Bax]

Hi Steve, Greetings from Merrimack, NH. Just a note on the rear master. There is a small spooge hole that must be cleared penetrating the main cylinder wall for the cylinder to function. The piston rubber assembly stack up is also crucial for proper pressure supply and makeup. There are a few good pics of these on this site I'll try to find but I suspect another member will have the link on speed dial and beat me to it.

---Bax

[cajun31]

'78 XS1100, rebuilt the rear master cylinder as part of a restore on a bike that has been sitting for 10+ years. Everything else is done on the bike, carbs sync'ed, petcock's replaced, front master cylinder replace. The bike runs great but can't get the rear brake working. Rebuilt the rear MC but having trouble getting it to pass the bench bleed as described below. With my finger over the brake hose, on the first pump I feel pressure and can let air esacpe. Then I cover the line again (with brake pedal still held down) and repump but don't get any pressure. If feels as if it is not getting any fluid through even though I took it completely apart and all the holes (even the spoonge) seem clear as I blew compressed air everywhere I could. Any suggestions what to do next? Should I replace the MC? If so, any recommendations as to which one?

Suggestions very much appreciated,
Steve

You DO need to "bench bleed" the master! If you don't, you will never get it to work correctly. With the brake hose off the master, put a finger over it while you depress the piston. Let the air escape, but seal with your finger BEFORE letting off the piston. Repeat until pressure and fluid come out, then hook everything back up and bleed the system.

You stated you made sure the spooge hole was open and functioning. Did you follow this maintenance tip? Particularly cleaning with the wire followed by inspecting the hole by shining a flashlight to see if you can see through the hole. Brakes not releasing? Clogged Spooge hole? - XS11.club Forums You stated you could feel pressure which usually indicates you have the push rod rubbers oriented correctly and that they are seated correctly. From what you describe I would have to agree with BAX that you still must have gunk preventing the pressure from releasing via the spooge hole. One other question... Did you replace your brake lines with stainless or are you still using the originals? The reason I ask is because 40 year old rubber brake lines can break down internally introducing debris into the master etc....

[jkorn]

Hey guys. Kind of a similar issue. Front brake works but does not release. Only. 1/4 inch or so of Handel travel. The plunger does not come back out. So you need to pull it in almost all the way to the handle bar.

t think this is a bad description. Anyone understand. Jeff.

[cajun31]

Hey guys. Kind of a similar issue. Front brake works but does not release. Only. 1/4 inch or so of Handel travel. The plunger does not come back out. So you need to pull it in almost all the way to the handle bar.

t think this is a bad description. Anyone understand. Jeff.

What all have you done concerning cleaning rebuilding? If you have either cleaned the entire system by completely disassembling etc... or for that matter have refreshed both the master and calipers with all new seals etc... then your problem may be the exact same thing as the OP. If you have not done those things then you need to start with a complete overhaul cleaning etc. From what you stated I am assuming the brakes work because you have been through them. In that case you are probably dealing with the same problem. Read the tip repair link I posted previously. That will get you going. There is also another tech tip under maintenance repair brakes that shows the proper way to put the new rubber seals on the plunger. They can sometimes not be seated correctly and do not create pressure. Again I don't think you have that problem because you stated the brakes are working.

[jkorn]

Thanks. I’ll give it a try. Couple of years ago I did a complete breakdown and replaced hoses with stainless. Bike sat for several years. Brakes were locked when I tried to move it but broke free.

bottom line I need to ride it more. Maybe I’ll break it all down again. Can’t believe I’ve had it for 42 years. Thank god the carbs are all good now.

thanks for the reply.

[cajun31]

Thanks. I’ll give it a try. Couple of years ago I did a complete breakdown and replaced hoses with stainless. Bike sat for several years. Brakes were locked when I tried to move it but broke free.

bottom line I need to ride it more. Maybe I’ll break it all down again. Can’t believe I’ve had it for 42 years. Thank god the carbs are all good now.

thanks for the reply.

Jeff... if the bike sat for that long the brakes definitely need to be gone through. Meaning completely disassemble the masters and the calipers. May not have to replace anything but definitely worth the inspection. The reason is the length of time and the fact that the brake fluid is hygroscopic... absorbs water. The gunk has probably built up around your caliper piston seals as well as in the plunger areas of the masters. At minimum a good cleaning and brake fluid replacement is in order. At the extreme ... and in my opinion not that extreme it is well worth replacing the caliper pistons with stainless if the slightest bit of pitting is found on them. Refreshing the rubber components is always a good idea as well. Fortunately you already upgraded the lines. Good luck on whatever you decide to do.

[cajun31]

This thread I had a few years back is an interesting read.... might want to consider the red rubber grease. Master cylinder and caliper rebuild - red rubber grease - XS11.club Forums

[steve112112]

You stated you made sure the spooge hole was open and functioning. Did you follow this maintenance tip? Particularly cleaning with the wire followed by inspecting the hole by shining a flashlight to see if you can see through the hole.....

Hi Jim, I did put a wire in the spooge hole to clean it but didn't follow up to check if I can see through it. I will do that. Also, in the attached picture, are the parts in the correct order? I lost track and not sure if goes plunger, rubber washer, cap, spring, or plunger, cap, rubber washer, spring. Also is there a metal washer that goes in there somehow. I have attached a picture of what I think/hope is the correct order.

[steve112112]

Hi Steve, Greetings from Merrimack, NH.

---Bax

Greetings and thanks for the suggestions. Small world, my daughter lives in Merrimack, NH! Nice riding out that way. I am just west of Portsmouth in Greenland.

[bikerphil]

The funny looking metal washer is in the wrong order in your picture, it goes between the outer rubber and the spring. The narrow end of the spring should fit onto the smaller end of the funny washer. The thin regular type washer goes on the piston before the outer rubber goes on.

[cajun31]

Ditto on what Phil said and I would suggest that you buy a rebuild kit for that master. The plunger is fairly rusted on the one end. The kits come with a new one.

[steve112112]

So, just to be clear (because I'm a bit dense), if I push a thin (<= 0.015") wire into the spooge hole I should have it come out into the bore of the master cylinder? That happens with the larger, lower hole, but not the spooge hole. Perhaps I am using too thick of a wire, will try and get something thinner. I tried shining a flush light up the bore and only saw light from the lower of the two holes.

[bikerphil]

A .010 guitar string will work to clear the spooge hole and yes it passes thru into the bore.

Here is the tech tip for clearing the spooge hole in front and rear M/C's...

https://xs11.club/forum/repairs/brak...ed-spooge-hole

[steve112112]

A .010 guitar string will work to clear the spooge hole and yes it passes thru into the bore.

Here is the tech tip for clearing the spooge hole in front and rear M/C's...

https://xs11.club/forum/repairs/brak...ed-spooge-hole

Awesome and great idea. I play guitar and should have some extra strings hanging around.