Topic: Brake Master Cylinder Dust Boot

[quidger]

The rebuilding of the master cylinder was not such an ordeal until I got to the very last piece to install - the crappy little spring ring that is supposed to hold the rubber dust cap in place.
I thought everything was in place so I installed and bled the system. Started getting small leakage from the lever so I knew that I needed to start again. The little spring ring that holds the dust boot in place had come off or was never on securely in the first place.
So after three hours of trying to get the ring to snap into place over the dust boot I notice that I had nicked or torn the rubber dust boot.
Of course, Honda only sells the master cylinder rebuild kits with all the parts as a set so I am SOL.
Two questions:
1) Does any one know where one might locate a dust boot or two?
2) Is there any thing that I can use that would repair the small nick in the rubber dust boot and sand up to brake fluid?

Thanks / 1975 CB550

[77cafe750]

might be able to help you out i will know tommorrow i might have some extra boots

[Frostyboy]

Uh oh! The dust boot is just that. It's not supposed to be a hydraulic seal. If fluid is leaking from there I think you may have bigger problems.

[MarkM]

Recently rebuilt a MC myself and my understanding is that if you have a leak that little dust boot won't do you any good in stopping it.  It's just there to keep to much crud from getting down in there and if all the crap in mine was any indication it doesn't work worth a $#&^!  Well I suppose over 35 years some stuff is gonna get in there. 
Did you check the pitting in the bore?  If it's to pitted fluid will leak past the plunget gasket and out between the washer and past the circlip.  Another thing is did you have the old one you took out to follow on how that plunger gasket goes on?  It's a tough nut to get over the end and all the way to the middle.
M

[quidger]

I did use the old parts as a guide to make sure that I got the new parts in the same as the old. So I am certain that I got the plunger and rubber gasket and circlip installed correctly. The bore did not look pitted or scored but maybe it was.

So if I am getting fluid coming from the brake lever area it is not related to the dust boot? If the plunger, seal/gasket and circlip are installed properly then brake fluid should not leak past the circlip. Is that correct?

Is the only purpose that the dust boot serves is to protect from dust, dirt and other crud from going back into and past the circlip and plunger?

If that is the case it seems as though I have an issue with a pitted and/or scored bore or the gasket seal on the plunger instead of an issue with the rubber dust cover.

Any thoughts?

[MarkM]

My understanding is the dust boot is just to keep crud from getting back in there and potentially jamming up the piston from being able to move inward.  Like a small pebble or something flying in there and getting wedged where in the brake lever wouldn't be able to push the piston inward. 
You're right, if you have the washer and the circlip installed correctly no fluid should leak back past it and out into the dust boot area. 
It's the rubber gasket that you had to squeeze onto the piston  that keeps fluid on the outlet or hose side of the MC.  If you look in the fluid cup, down the big hole nearest the lever and pull the brake lever you'll see the middle of the piston between the gasket and the end with the star on-way holes.  So the only way fluid can go back out towards the brake lever is to get past that rubber gasket.  If it is in good condition, assuming it is as you said it came from a rebuild kit correct, then only thing I can think of is pitting in the walls of the bore allowing fluid to leak by.

[MarkM]

Wanted to add in the FAQ on brakes, or maybe it was another brakes post, I read where you can use a #12 flex hone in the MC bore to clean it up.  Perhaps put "flex hone" in the search and see what pop's.  I used a small square of red scotchbrite wrapped around a 1/2" delrin rod to clean up the bore on mine.  Didn't have any real big pitting, just some surface stuff that came out with 20 or so strokes.  Used a dental pick set to scrap off some rust, or I think it was rust, at the very bottom where the one way valve under the spring goes nearest the hose end. 
Will check this post tomorrow, best of luck.
Mark

[quidger]

Thanks for the input. I will totally disassemble the master cylinder guts again and try to examine the bore. I will get this solved.

[MarkM]

One other issue I had with mine was reassembly.  When I would push the piston in/down and look closely at the washer behind the circlip it would drop with the piston.  Thus I didn't have the circlip fully seated.  Wouldn't have know this though without pushing down the piston and looking.  It's a tough bugger in there, maybe check that before totally taking it back apart.
Good luck.

[Duanob]

There are plenty of dust boots on EBAY for < $10. I'll be buying one myself.

Curious what brand of master cyl rebuild kit you used? I'll be buying one myself. So far I'm not impressed with K&L brand seals (forks) if you used the same brand and it's leaking then no more K&L for me!

[MarkM]

Duanob,
I used a Honda kit.  Kinda expensive but figure it's one of the parts in the group of parts that keeps me from running into stuff!!

[quidger]

There are plenty of dust boots on EBAY for < $10. I'll be buying one myself.

Curious what brand of master cyl rebuild kit you used? I'll be buying one myself. So far I'm not impressed with K&L brand seals (forks) if you used the same brand and it's leaking then no more K&L for me!

I used the OEM Honda. Bought it from Service Honda. Think it was about $26.

Okay I disassembled the master cylinder again. It does look as though there is some serious pitting and a few significant scratches in the bore. I was going to pop for a 12MM flex hone but I cannot find anyone locally that stocks one. I found a couple on line for about $20 but if I can get as idea or two from some forum guys on another approach I'll try that first.
What other methods or make shift tools have any forum members used to hone the inside of the bore of the master cylinder?

Thanks

[Frostyboy]

Years ago I used to get a piece of pine doweling of a slightly smaller diameter of the bore, cut a slot in one end about 1½". Get some fine grit wet & dry paper, cut a strip 1½" wide, slip one end into the slot & wrap the paper around the dowel until it was just a snug fit into the bore. I used methylated spirits as a lubricant. Make sure you don't spin it in the one spot without moving up & down the bore for the whole length of the bore.
Cheers.