Brake System Overhaul Yes or No

[Jeff in W.C.]

So, my plan was to restore my 1988 R100 RT after purchasing my GS. However, I spent a bit too much on my GS and now I have to put off the restoration. The bike really should have some repairs to make it more reliable so I can ride it until I can get to the restoration. I have some questions regarding the brake system.

The bike is ±35 years old. What are people’s thoughts regarding rebuilding the calipers and/or master cylinder? My thought is to disassemble and replace all the O-rings and gaskets at a minimum, and anything else that may seem out of spec. After ±35 years, I would think the O rings and perhaps the gaskets at the end or nearing the end of their economic/useful lives. However, I’m unaware of any major issues—the brakes seem to work OK at the moment. Should I be proactive here and rebuild them, or wait and repair as needed down the road? FYI, I do plan on replacing the brake lines with braided stainless lines.

[gspd]

You can spend it now, or you can spend it later.
If the brakes are performing satisfactorily, no dragging, no leaks, you can postpone the surgery.
Has your brake fluid been changed regularly?
If not, changing the brake fluid, like when installing new brake lines, can push old master cylinder seals
past their normal travel in the bore and dislodge rust and dirt, which will cause problems.

1990 gspd, 550,000km
My front brake was dragging a bit.
I could've simply removed a piece of crap that was partially blocking the return hole in my master cylinder.
That would have fixed it.
Instead, I installed a new master cylinder rebuild kit and fluid container, caliper pistons and seals.
My original fluid container was a bit yellowed with time, and some fine spiderweb cracks were appearing in the plastic.
My (also original) composite caliper pistons also had some fine cracks around their edges.

It feels great now, good as new,
but what feels even better is knowing that I won't have to do it again in my lifetime.
Unless I live (and ride) to over 100 years old.

My doctor told me a decade or so ago that I'd live to over 100;
He died three weeks later.

[Jeff in W.C.]

GSPD, I'm less worried about money as I am about riding the bike with good safe brakes.

I have a tangent question. BMW upgraded the master cylinder from a 13mm one to a 14mm one in April 1989. Would it make sense to upgrade to the 14mm one rather than rebuilding the 13mm one. Looking at the parts fiche, it does not look like there's any different parts in the system other than the larger master cylinder.

[gspd]

GSPD, I'm less worried about money as I am about riding the bike with good safe brakes.

I have a tangent question. BMW upgraded the master cylinder from a 13mm one to a 14mm one in April 1989. Would it make sense to upgrade to the 14mm one rather than rebuilding the 13mm one. Looking at the parts fiche, it does not look like there's any different parts in the system other than the larger master cylinder.

If your discs and pads are good, there are no leaks, there is no sign of dragging or vibration, the brakes feel good during hard braking, and if the lever stays firm and holds pressure when you apply your full force to it for 30 seconds, YOUR BRAKES ARE SAFE.
Master cylinder and caliper seals never fail suddenly resulting in a no brake situation; they deteriorate slowly over time. Sponginess, leaking, or dragging are the first symptoms and will appear well before a sudden failure. Brake lines however, whether flexible or metal piping, can cause sudden and total brake failure when they let go, so it's best to assure that all your lines are in top shape. It's a good idea to replace (or regularly test by pressurizing it with your full strength for 30 seconds) any flexible line that's decades old, and carefully inspect the solid pipes for corrosion or any sign of weakening.

AFAIK, BMW (airhead) master cylinder sizes vary from 12 to 17mm depending on the type of caliper used and if there is one or two of them.
If there is no specific problem, if your brakes have been fine for decades, stick to the OE size.
All things being equal, a bigger master cylinder piston displaces more fluid, so the lever will have less travel but require more force to produce the same amount of pressure at the caliper, but 1mm either way won't make a huge difference.
https://www.euromotoelectrics.com/category-s/2929.htm

[Jeff in W.C.]

AFAIK, BMW (airhead) master cylinder sizes vary from 12 to 17mm depending on the type of caliper used and if there is one or two of them.
If there is no specific problem, if your brakes have been fine for decades, stick to the OE size.
All things being equal, a bigger master cylinder piston displaces more fluid, so the lever will have less travel but require more force to produce the same amount of pressure at the caliper, but 1mm either way won't make a huge difference.

The monoshock RT's from 1988 to 1995 used a 13mm master cylinder up to about 04/1989 then switched to a 14mm one which was used to the end of the run (at least it seems that way based on the parts fiche). As such, both are technically OE size. For some reason BMW switched to the 14mm one with all things else being unchanged. I would hope it was for improvement/better braking purposes and not because it was less costly or it was more convenient, but I'm just guessing here. The 14mm one is also a little bit less expensive--for whatever that's worth.

[gspd]

[ The 14mm one is also a little bit less expensive--for whatever that's worth.

Is there a specific negative issue with your present 13mm setup that you are expecting to resolve or improve by going to a 14mm? Is it too spongy or does it have too much travel when properly bled? Or was it always OK (until now)?
Keep in mind...
If you go to 14mm master cylinder, you need a complete unit, over $200.
If you rebuild your 13mm master, you just need a rebuild kit, about $70.

[Jeff in W.C.]

Is there a specific negative issue with your present 13mm setup that you are expecting to resolve or improve by going to a 14mm? Is it too spongy or does it have too much travel when properly bled? Or was it always OK (until now)?
Keep in mind...
If you go to 14mm master cylinder, you need a complete unit, over $200.
If you rebuild your 13mm master, you just need a rebuild kit, about $70.

I looked under my master cylinder for its size and I noticed some seepage, so it's probably good to do a rebuild on it. If things are good otherwise with the 13mm one I have, then I'll just rebuild it. However, if it needs replacing, then I'll probably "upgrade" to the 14mm. While things are apart, I might as well rebuild the calipers, then they'll be good for another 35 years.

[barryh]

What I would do as a minimum is inspect the caliper pistons for corrosion. You can do a tolerable good job of this without removing them from the caliper. Remove the pads and dust seals then pump them out but not so far that they leak fluid. You will be able to see if there is any corrosion on the part of the piston outside of the caliper seals and decide if they need cleaning up or replacement. The other thing you are likely to find is that one piston moves before the other indicting some stiction and need of cleaning/lubrication, Lubricate the pistons with red rubber grease or one of the newer silicone based brake greases to keep corrosion at bay.

[SteveD]

https://www.bmbikes.org.uk/viewtopic.ph ... p.#p211044

[gspd]

I looked at that link. All good, except you can't use screwdrivers to pry the pistons out of the calipers when you have the later composite (not metal) type pistons, the edges will break off.
The good thing is that the composite pistons never rust or corrode.
The bad thing is that they can develop spiderweb-like cracks around their edges with age.
EME sells OE Brembo pistons. BMW doesn't.