Good questions. Basically ALL 40 year old master cylinders (m/c) need some attention ! The reason is that DOT4 brake fluid attracts water, which causes all the rust, which causes all the rubber seals to wear, which causes all the leaks. Good plan too, but let's not jump the gun. My suggestions...

1) Remove the m/c from the bike. This means disconnecting the steel line (11mm), unplugging the 2 brake lamp switch wires, disconnecting the op cable, and undoing the steel band clamp. Be sure and wash any area with hot soapy water that the dripping brake fluid falls onto.

2) Use a pair of "snap ring pliers" to withdraw the C-clip holding the piston. Use a metric socket to remove the reservoir. Unscrew the brake switch. Wash/scrub all these parts in hot soapy water.

3) Inspection of the m/c bore is usually impossible until all the rust is removed from the first 1" of the bore. This is best done with a small length of medium grit (~380) emery cloth wrapped around the end of a 4" length of wooden dowel. Use a drill motor to turn the dowel and thus remove the rust and restore the bore. There are always minor pits, but you are looking for divots wider than 1 to 1.5mm. The absence of bad pitting means you can use the rebuild kit; the presence of deep divots means you need a whole new m/c.

Other Thoughts
• The seals are leaking becasue the DOT4 brake fluid wasn't flushed on an annual basis. If this was true for the m/c, then it is certainly true of the caliper. Your caliper will work better if it is also inspected. A simple disassembly, washing, drying and re-assembly is usually all that's needed.

• I urge you to consider replacing the flexible black rubber hose at this same time, and replace it with what's known as "stainless steel brake hose". The OEM hose last about 15 years, the stainless (which are Teflon lined) last a lifetime. Your braking will become MUCH more efficient and effective with a modern brake hose.

• You must inspect the wiring just below the m/c on the LH side. If any of that wiring has the brake fluid on it, or a gelatinous, gummy residue of old brake fluid, then it must ALL be washed/scrubbed in hot soapy water (are you starting to see a pattern here ?). The small 1x1x1 cube is the starter relay and a major electrical path passes through that relay socket. If the caustic DOT4 brake fluid has attacked the brass contacts of the relay OR the relay socket, then you are in for major electrical issues.

• A suggestion: If you are weary of DOT4 brake fluid attacking your painted surfaces, electrical connections, forming rust on your master cylinder walls, causing flexible hoses to fail, attacking your skin, or the giant aggravation of replacing it once a year, then you might want to research an alternate fluid called DOT5 Silicone. Now you must NOT mix DOT4 and DOT5 Silicone, so the perfect time to convert over is when you already have everything taken apart and cleaned. And it is not suited for ABS, but your /7 doesn't have ABS. Both these caveats fit your present situation.

I work on motorcycles professionally, so when I get home the LAST thing I want to do is fiddle with brake fluid ! That's why I've been using DOT5 Silicone in my personal Jap and European bikes (including my present 1979 /7 [Ate] and 1988 RS [Brembo] ) since the mid-1970's.

Hope this helps !

Thank you so much for the in depth reply! I will get this started soon!

You're certainly welcome. These are all steps I've taken when bringing a 1979 /7 out of long term storage by the PO.

Not sure what kind of workshop setup you have access to, but compressed air is VERY useful when working on parts of the hydraulic brake system. Not only can you clean and dry with compressed air, but it is sometimes the only way to get the tight-fitting pistons (both m/c and caliper) to pop out during disassembly.

Note here that "disassembly" is where 99.9% of the "work" lies in doing hydraulic brake system rebuilds. So without access to compressed air, this work can be almost impossible.

Generally, brake work is a great way for owners to save big money, and at the same time make upgrades that can become truly life-saving. And one of those changes that gives the most "bang for the buck" is modern flexible brake hoses. Another is modern brake pads.

Before disassembly do this.... Wrap your hand around the black rubber brake hose. Then squeeze the brake lever as hard as you can. If you feel the brake hose swell or expand in your hand even the tiniest bit, then your brake hose needs replacement. What's happening is that the braking power created by the m/c is being partially diverted to simply expand the hose. Obviously, that hose expansion is wasted power. When you need to stop, by God you want all 100% of that power concentrated on bringing you to a stop. The last thing you want is 10% or 20% of that power siphoned off to simply expand the hose, but that's what's happening with rubber hoses. Modern "stainless steel" brake hoses fully contain the pressure with an outer steel braid, thereby directing a much, much greater portion of the hydraulic pressure to the caliper(s). This is not an academic exercise, this is increased stopping power you can really feel. The end result is that the safety of the "2-finger braking" riding position becomes much more feasible for the average rider.

Hope this helps.

I have the MC apart and have ordered spieler brake lines, I have to get the top part separate. I am also going to order a rebuild kit- the inside of the MC seems good. It was very dirty and the (piston?) part- the had some rust so obviously not much happened with it before me. I am trying to get the electric for the lights to come off the MC it is super tight. Probably am going to need a vice, I did spray some wd-40 on the threads to try and loosen it. Once I get that off I plan on washing thoroughly and cleaning the MC. Then hopefully the re build kit would have arrived. The reservoir container had a gasket that sat under it and attached to the MC- will that be in the rebuild kit- I an having trouble finding one, it said to replace when taken off. Thanks again!

I have the MC apart and have ordered spieler brake lines, I have to get the top part separate.

Good choice on the brake lines. What "top part" ? Do you mean the reservoir or the reservoir cap ?

I am also going to order a rebuild kit- the inside of the MC seems good. It was very dirty and the (piston?) part- the had some rust so obviously not much happened with it before me.

The DOT4 fluid attracts a lot of water. The water causes rust. The rust is quite common on machines with deferred maintenance. The calipers won't be in much better condition. This is why DOT4 should be replaced every year on ANY motorcycle.

This is also why I suggested you research DOT5 Silicone. It doesn't attract water, doesn't cause rust, doesn't need to be changed as often, doesn't damage paint, doesn't cake up and cause m/c issues, etc, etc. In short, if the last owner had converted the fluid, then you wouldn't be having all this "fun" right now.

I am trying to get the electric for the lights to come off the MC it is super tight. Probably am going to need a vice, I did spray some wd-40 on the threads to try and loosen it. Once I get that off I plan on washing thoroughly and cleaning the MC. Then hopefully the re build kit would have arrived.

Do you mean you're having trouble getting the 2 connectors to slide off the brake lamp switch, or do you mean you're having trouble removing the switch itself ? The connectors slide straight forward, toward the front of the bike. The switch is threaded into the m/c body and does NOT absolutely have to come off for a good rebuild. If it's not leaking at the threads, then leave it alone. (You can mail me the $50 I just saved you later. 😛 )

The reservoir container had a gasket that sat under it and attached to the MC- will that be in the rebuild kit- I an having trouble finding one, it said to replace when taken off. Thanks again!

You should get all new o-rings, seals and cups with the kit. Everything it takes.

Cool I hope the seals come with the rebuild kit I got from Bob's. And I cannot get it off from the master cylinder , it disconnected nicely from the brake light cord. I will wrap it in a baggie and zip tie it so it stays nice and dry.

Cool I hope the seals come with the rebuild kit I got from Bob's.

You can always call and ask Bob's. They're very helpful.

And I cannot get it off from the master cylinder, it disconnected nicely from the brake light cord. I will wrap it in a baggie and zip tie it so it stays nice and dry.

► I'm still assuming that "it" is the brake light switch.

► The best way to clean the m/c and all the parts is to wash/scrub them in hot soapy water. Then you can warm them up with your hair dryer, or a hot air gun, until all the water evaporates. When your rebuild kit arrives, all the rubber parts of the piston should be moistened with brake fluid using a Q-tip (or similar swab) before assembly.