58 rear master cylinder

[PanheadBob58]

I have tried rebuilding my rear master cylinder with 2 different kits now and both did not build up pressure and also seemed to allow fluid seeping out onto the boot. So I have a couple of questions: the first would be is there a company out there that sells a rebuilt original master cylinder as a complete unit, the second would be is a reputable company I would use to rebuild (probably sleeve the cylinder) my current master cylinder, and the third question would be is that I had bought an aftermarket master cylinder that has a the fill cover noted as using Dot5 only but can I use Dot3 with this master cylinder which is described as an aftermarket replacement for a factory part no. 41761-58B. (V-Twin 23-0750) Thanks for all replies.

[RooDog]

Do you have a cast iron master cylinder?
They are very simple: There is a 3/4" rubber cup, and an O ring. The coil spring is also a potential wear item. So, other than the condition of the bore, there is not much that can affect the function of a conventional MC.
You did install the derby hat shaped check valve and rubber covered seat washer in the bore before anything else? Assuming drum brakes.
I prefer DOT 5 fluid as it does not absorb water like DOT 3 does. That absorbed moisture helps promote rusting inside both the master cylinder and the wheel cylinders.
So if the bore is not pitted, and cleans up with a brake hone, then most any kit with good quality 3/4"
rubber parts should work if assembled properly.....
....RooDog....

[PanheadBob58]

I'm glad that you have answered one question I didn't ask, and that was where the rubber covered washer was supposed to be placed in relation to the spring and hat shaped cup seal, when I diss-assembled the master cylinder I did not find this part as I laid out the parts in order of removal, so I installed from the kits only the parts I found to be removed, I could not find a diagram anywhere that showed exactly where this rubber covered washer should be placed, so I'll take it apart and install this washer before giving up. I will note that the original piston (the piston that was removed before rebuild) did have some minor wear marks. One of my questions is if I found a company that is offering the re-build service and I wanted to install temporarily the V-Twin master cylinder until I received my original master cylinder back from the shop, can I use DOT3 instead of the DOT5 that was called out and stamped into that filler cover as a requirement of proper use.

[PanheadBob58]

yes this is the cast iron master cylinder, forgot to mention in the last reply. I want to keep this because it just looks better and more original.

[Robert Luland]

If you look down the bore with a flashlight. Over years the seal digs a grove down by the front of the bore. Once this happens, there there's no saving it. The seal will by-pass. The bore is not round anymore. I have always run dot 5. Never had a fading problem. Bob L

[Chief3Kicks]

I have used Apple Hydraulics in NY for various antique automotive MC sleeving and have never had any issues with their quality or service. I'd imagine they could sleeve about anything. http://www.applehydraulics.com/

There could be a compatibility problem with DOT 3 relative to the seal type/design in the new MC, but I doubt it. It has been my experience that glycol based fluids are will not harm rubber parts. I'd hope any MC seals made today would be compatible with DOT 3/4 or DOT5 fluids. But....we know there can always be an exception. If you try it proceed with a little caution.

[RooDog]

One other thought: DOT 5 will not harm paint like DOT 3 will. I have used DOT 5 exclusively in my motorcycles since the late 1970s after seeing how the DOT 3 often turned to a gel in the handlebar master cylinders over time on many customers bikes....
....RooDog....

[blewcrab]

Just use the after market dot 5 ms and exchange the cap with the old ms that says use dot 3 .....worked for me !

[fenderguitarleo]

If you decide to use DOT 5 fluid which I also recommend, just make sure all the brake lines and cylinders are flushed through with the DOT 5, it wont mix with the old DOT 3.

Leo

[PanheadBob58]

I thought I'd give an update to my master cylinder issue: The final end solution is that I used the new aftermarket master cylinder that came shipped with a filler cap suggesting the use of DOT5 brake fluid only but I inspected the master cylinder prior to installing and it was dry on the inside so I went ahead and just used the DOT3 which is what was already flowing thru-out the rest of the brake system, but I did need to replace the brake light switch. So as I stated in my earlier post I at first didn't see the rubber coated bushing but once I used a bore sight light I could see that washer sitting down in the body, also I honed the crap out of it and re-installed the rebuild kit, bled the crap out of it and rode the bike and the brakes felt good but the brake light never came on so I was mis-led with the thought that the reason for the brake light not coming on was the master cylinder wasn't building enough pressure (even though my test ride told me different), so the brake light issue led me to installing the aftermarket master cylinder and then re-bled the system and again test rode the bike and the brakes worked just fine, BUT again still no brake light, so I replaced the brake light switch and presto just like that I had a brake light, I wish I would have changed that brake light switch prior to installing the aftermarket master cylinder but I'm planning on leaving that master aftermarket master cylinder on the bike for now, but I like keeping as many original parts on the bike as possible so in the late fall I'll switch out to my re-built original master cylinder. I did however re-install a chromed cover the covers almost the whole master cylinder that I pulled off when I had first bought the bike, so actually it would be real hard to see that I am not using an original type master cylinder. Thanks to all that replied with help. So the bottom line is a $18 part cost me an extra 4 hours of work and grief.

[Panacea]

Motorcycle maintenance is never a waste of time.

[RooDog]

I use a Stop light switch from a 1974 VW Super Beetle....