Master cylinder leaks, screws stripped, replace or rebuild?

First-time poster on this excellent site! Hope you guys can steer me right...

Summary of my problem:

Bought a 1982 XJ750 and the master cylinder was wrapped heavily with electrical tape and a bungee (always a good sign - but this was my first bike so what did I know?). The owner had the handlebars angled in quite a lot so I adjusted them back outward to be more straight. Long story short, after a few months the brake fluid slowly leaked out and I started to notice some sponginess. Now it leaks regularly, even if I try to re-tape, use a c-clamp, etc. to hold down the cover firmly.

The master cylinder on my bike is horizontally tilted-down (which is to be expected for a 750? The haynes diagram shows level orientation, so I wonder if PO maybe replaced the stock one with a 650 M/C); with my handlebar position it will NOT get level so the fluid is always up against the gasket unless I angle the bars back awkwardly.

Is it worth trying to rebuild? Has anyone ever attempted to tap new screw holes? There is an aftermarket setup on ebay for $50 that says its for a XJ750, is level, and has a 14MM piston bore (which I know is not quite up to snuff with the OEM 16MM bore and i'd lose some braking power):

http://www.ebay.com/itm/140766865295?ss ... 151wt_1141

Almost forgot to mention I DID notice the "old faithful" when operating the brake with the m/c cover off, so I am sure my main seal is shot and needs replacing.

I'd really like to just buy a new one, but I would like to avoid spending $300 for an OEM setup; I'm not afraid to rebuild the m/c but I am worried that when I do I still will not be able to control the leaking which annoys me so (my beautiful paint!!!)

Aftermarket worth a try? Am I psychotic to consider downgrading to a 14mm master piston bore from OEM 16mm? Should I just rebuild and tap new screws?

Thanks in advance!

Oh and wtf is 'bobber'?

 

Seca or Maxim? The 750 Seca has a master cylinder behind the headlight, so if it's a seca, then the MC is definitely not stock. If it's a Maxim, then the handlebars might not be stock either.

"Loosing some braking power" isn't like losing a little HP off the top end. No one really "needs" to drag race on the street, but there will be plenty of reasons for you to stop in the shortest distance possible on the hairy end of your tire's traction capability, and you don't want your brakes to fail you then.

Chacal has all the rebuild parts needed. You will likely spend $300. Accept it, brakes are not worth skimping on, you may pay for it with your life. You can always buy a used MC and rebuild it, if the screws in your resevoir are stripped. You may just need a new diaphram or gasket on top (in addition to the rebuild kit)

 

Re: Master cylinder leaks, screws stripped, replace or rebui

Its a maxim.

 

Re: Master cylinder leaks, screws stripped, replace or rebui

Sound advice. Might as well do calipers and lines while I am at it

 

From the sound of things, you have either: a Maxim M/C and non-original bars;

or a non-Maxim M/C and original bars. I suspect the former.

The original master cylinder had the reservoir at an odd angle so it would sit horizontally with the original "buck horn" handlebar. So one of yours (M/C or bars) is not original. From the sound of it, it's the bars.

If you need a "level" master cylinder for a bike with dual discs, then the one from a 650 Seca, 900 Seca or XJ700 would be ideal. You might be better off to find one of those and rebuild IT.

Or check with chacal on aftermarket options.

 

I just came to the realization that a decrease in piston bore size will actually INCREASE the braking force for a given lever pull force (but will also increase the stroke length). See the following for explanation:
http://www.airheart-brakes.com/pdfs/Mas ... ection.pdf

Now it seems to me the only negative with the 14mm aftermarket m/c will be the mirrors dont fit (I think adapters exist). Since I will be able to achieve more fluid pressure, I will definitely be getting the SS braided lines; I checked yesterday and, as expected, my rubber lines are stamped February 1982 (yikes!)

Opinions? Thanks for the previous responses too

 

To start off with. The 82 750 Maxim used the modular adjustable handlebar just like the 750 seca, and the M/C body should run parallel with the handle bar. The 650 M/C used an angled mount to work with the rams horn style handle bar.

Stepping down in size on the piston from 16mm to 14 mm is not advisable. It is not the fluid presure that you need to be concerned with. The larger piston moves more fluid volume to operate the dual caliper setup. The smaller piston may not give enough volume for the brakes to operate properly.

With the stripped out screws you are better off finding another M/C and rebuilding it, or go with a new M/C that matches the same bore, and stroke as the stock M/C.

The SS braided line are a deffinate improvement in providing better braking. The old rubber lines flex, and bulge when the brakes are applied. The SS lines will not. This makes all the fluid pushed by the M/C work at the caliper.

While you are working on the front brakes. Make sure you rebuild the brake calipers also. It is highly unlikely that they have had any attention from the PO, and are running on hopelessly outdated seals.

A bobber is used in fishing. It lets you know when there is a sucker trying to steal your worm. :lol: :lol: In cycle lingo it is a poor excuse for hacking up a bike, and painitng it flat black

Ghost

 

Re: Master cylinder leaks, screws stripped, replace or rebui

Thanks Ghost. I do have the modular bars (which I thought was stock) so most likely it is a m/c from a 650 (or other angled one).

Definitely planning a caliper rebuild as well.

 

I used one of those from eBay and it worked fine, but you have to put some connectors on for the brake light switch and you'll never find a rebuild kit for it.

 

Polock, does yours have dual front discs?

 

82 750 maxim, yes dials. Your going to have to replace the old lines, with the smaller bore there's not any extra to waste swelling those old rubber lines.
You should get new lines for whatever MC you get

 

Re: Master cylinder leaks, screws stripped, replace or rebui

Sweet, will give it a go, thanks

 

I dont quite understand your original post. You need to rebuild your master cylinder and cant get to it because the cover screws are stripped? If its just the screws in the cover you could still get them out.

I just did this job yesterday and with a little patience and thought they came out. Assuming the screw head is still on of course! You might have to spend a few bucks on some tools. Let me know if you wanna know how I did it.

 

Re: Master cylinder leaks, screws stripped, replace or rebui

No the threads which the screws go into are stripped so I cant get the cover down tight to prevent leaks...

 

Ahh. Makes sense now. sorry.

First inspect the threads all the way down the holes. There might be some thread left in there. If so switch to a longer screw. If there are only a few threads left then I recommend getting a hold of a bottoming tap. The hole thread size of the OEM M/C is M4x.7. Make sure its a bottoming tap! Use the bottoming tap to cut more thread as there is more meat down that hole for it.

Or if your good with a hand drill you could drill and tap up to an M5

Size...Pitch........tap drill in inches
M5,,,, x .8,,,,,,,,,,#18 (.169)

Check local hardware stores for taps. BTW. If you decide to drill and tap to an M5 you do not need a bottoming tap. Just one youll likely find at a hardware store. If you have a choice between a plug or taper, go with the plug tap for this job.

To do the job.
With the cover on take a small drill bit or anything else and place it in the hole.

Mark that depth on the small drill while its in the hole.

Transfer the measurement to the #18 tap drill by wrapping tape neatly around the tapdrill. This will let you know when youve reached the right depth.

Now place the cover on the M/C making sure the holes are lined up and clamp or strap or do whatever to hold the cover in place.

Drill hole keeping the drill as perpendicular as possible. Dont going flying through or anything, just nice straight pecks til you reach your measurment. A drill press would be ideal, but aluminum is easy. you want a clean straight hole which shouldnt be a problem with a steady hand.

Next, Tap hole with M5 tap. Fill the hole with WD40 (dont skip lube). Keeping the tap as straight as possible give it positive down force while turning it in. When tapping you have to break the chip your cutting so remember to go counter clockwise ever half turn or so. A tap wrench is ideal, but for two holes not a necessary expense. Use a small cresent wrench. Remember to keep the tap as perpendicular as possible when starting the threads and break the chips every half-full turn after youve started the hole.

Now install new Stainless M5 flat screws.

Note: the cover can be on or off while tapping. Just watch the chips. If it is off while tapping, remember the hole in the cover has to be enlarged inorder for the screws to get through it. personally, I would tap with the cover on.

really not a difficult thing for you to fix. you might have to carefully modify the holes in the diaphram, but the M5 might go through that ok too. keep an eye on that.

Enjoy.

 

Re: Master cylinder leaks, screws stripped, replace or rebui

Thanks! Altough I have discovered other problems and will definitly need a new m/c, this was good info since I have never tapped new screws in before and was curious about it...