Bringing 74 R90S back to life after sitting 30 years

it's brake master cylinder time... what an unusual layout with BMC under the tank
those Germans must have stayed up late to come up with this one.

didn't want to order a BMC kit until I could inspect bore for pits too deep to hone out.
looks like I lucked out, bore is in pretty good condition. can't same the same for my poor frame.
brake fluid destroys the paint. what a mess underneath.

bleed off port was completely plugged, not allowing brake fluid to flow back releasing calipers.
hence why front brakes was locking up.

When building one of the most beautiful motorcycles ever created it makes perfect sense to move the ugly specimen cup to a hidden location. Compromises sometimes need to be made.

I think of ATE master cylinders as sort of a brake fluid volcano that could errupt at any time. I don't think I've ever seen one that wasn't leaking.

You've got to remember these hydraulic brakes were the cutting edge. It was all drums before that. Maybe this was all Magura or ATE came up with - I have no idea who built these master cylinders. It may have been BMW, but considering all the other brake parts were either made by ATE, Brembo, or Magura I wouldn't expect these cylinders to have been built by BMW.

At any rate, they do seem to last a long time and work fairly well when maintained. Change the fluid every year - it's an easy and quick job and considering the cost of these dumb things anymore, well worth it. Pull the tank every now and then and inspect the thing - pull the rubber boot and take a look, as fluid will collect there first before erupting onto the frame.

It can be a bit of a pain cleaning the rust off the frame, but it's one of those jobs - somebody's gotta do it. And it's sooo satisfying when it's all nice and purdy again.

not bad as I thought ... it could be worst. could have been looking at a new brake master cylinder ($400 eeek) instead of $65 ... insides ID is 15.87 mm or 5/8in (.6250). this bike uses the smaller 16mm bore to give leverage ratio of about 28 to 1, instead of 21 to 1 ratio for 17mm bore. Maybe those German engineers weren't so dumb after all

inside seals sure look like standard ATE style brake parts. sure be nice to match up seal kit with a more common/cheaper rebuild kit. no reservoir cup seals are available from BMW. so matched up a few nitrite HVAC seals that hopefully will not leak.

That setup combines cables which I like and hydraulic which I like into a combo that I don't like. I have always called it the worst of two worlds. They were good brakes for their day. Cutting edge? Not if you include some Moto Guzzi and Ducati models for starters IMO. Personally, I would convert it to a normal setup before I spent much money on it.

It is said that converting to a handlebar MC is an improvement but I've been told a new cable also helps. I did a new cable some years ago and I think it was, as said, an improvement. Was it just wishful thinking?

G'day Cy.

I've just been through all that with my /6 with dual disks. My bike *seemed to be* a low mileage unmolested bike when I bought it 7 years ago.

Just a few things I found; you probably know all this already...makes sure the holes in the master cylinder are not blocked. Some of the orings as you have mentioned are not available; I just used mine over again. YMMV

I also replaced the hoses, and rebuilt the calipers. I can't tell you much of a huge difference the caliper rebuild made....all the seals had swelled and the rubbish inside the caliper body was unbelievable!

I also used a big syringe to suck the fluid out via the bleed nipples, but I see you have a garage full of tools so you might have something better anyway.

When doing a final bleed I removed the calipers, with a wedge between the pads, and got my son to hold the caliper high as he could above the mudguard to get the final small air pockets out...worked like a beauty!

Finally, as I have slowly been refurbishing my bike over the years as time/funds permit I found one thing out, many times: Anything rubber/plastic etc after all these years you have to assume is past it. For example I was fiddling about with a new top brace/clamp. The headlamp ear rubber grommets I thought I could get away with reusing. But sure enough they were perished when I had them out. Battery box rubbers, seat bumpers, even the rubber bleed nipple caps...the list goes on!

All the best. A great looking 'survivor' R90S you have there!

OZ.

oz_airhead, probably should change my screen name to "toolmeister" aka the Hack

cleaned up frame followed with black paint.
not worried about a perfect paint job under tank. so long as it stops corrosion on top tube.

onward to the brake master cylinder. got it all taken apart and cleaned up.

all sorts of junk inside... cleaned out with brake cleaner, then used carb cleaner to remove residues.
there's two holes from tank to cylinder. pay special attention to tiny hole under tank. both must be clear for BMC to work properly. larger hole feed fluid into cylinder, tiny hole is the pressure relief function. if the hole is plugged fluid cannot back up, locking up calipers. there's another set of pressure relief holes directly under front seal. make sure those holes are cleared. those holes are pressure relief valves. allowing fluid to bleed back after pressure is released.

sorry got lazy and didn't take pic's of honing. take 120 grit emery cloth and a wood dowel. proceed to hone inside of cylinder. make sure there is no deep pits, which will cut up new seals. you may get by with shallow pits, so long as transition by seal is smooth. used new nitrite Orings seals under tank.

lucked out and bore honed out real nice and smooth with zero pits. inspected old seals, which was in amazing condition with no damage. since cylinder was not leaking from bore, decided to re-use old seals until new seal kit makes in the door. will rebuild BMC along with calipers when kits gets in.

will post a detailed tutorial on how to rebuild a brake master cylinder then
really want to get motor started to see if I've got a working engine and transmission.

brake master cylinder all finished and ready to install. will let paint dry overnight
note: use a dab of silicon grease on both ends of activation rod

+1000

I'd had a BMW car ('76 2002) many years before I got a boxer. The first time i took the '77 RS tank off and saw that master cylinder, i thought "This is where the Car side of BMW has infiltrated the bikes."

And yeah, these hyd brakes were new to the industry. So kudos on early engineering.

But a bike needs a handlebar-mounted reservoir, period. Not hard to retro-fit w/ all factory/Magura gear either to any /7 and earlier


Looking good, CY

Those are ATE master cylinders, aren't they? I seem to recall that now.

It's true, the bar mount master is a more direct system - less parts and vias.

But, when I converted the R90 to dual discs I didn't know the dual master came with a larger bore. Not knowing that, I had the single (I believe 14mm master) bored and tapped to accept a second brake line.

That made all the difference in the world - those brakes really work!

yup it's a cylinder made by ATE. strangely enough 16mm bore size is listed for single disk. dual disks lists 17mm bore size. what size bore does R90/6 have for brake master? this bike definitely has a 16mm bore (15.87mm or 5/8in actual)

converting to handlebar reservoir would require butchering up electrical controls. not going to happen. especially when current system works. maybe not state of the art today, but I suspect more than adequate for my braking needs.

new magura grips are on the parts list

I have two under the tank MCs that I picked up somewhere. One I know is from a '77 R100S the other one I don't know. But both have the cast in # "17". I've not measured them. I think a "16 should work great and even a "15" would be OK but eventually you run out of grip travel and during hard breaking it can not work so good. The two calipers are "40"s and they take more fluid to work. Although I've heard of people using the stock MC for a single disk on a dual set up I don't think I would do that. The single is a "13" I think. It might be "14".

If your "16" ever pits up on you you can convert to "17" easily with out sleeving.

I also painted the frame in the same place ten years ago. Time to do it again and I'm converting to Duals, so there.

Charlie

The 90/6 single disc came with a 14mm master cylinder.

One thing you need to realize is that the mid seventies was the beginning of disc brakes for BMW motorcycles and they hadn't worked out the ratios yet. As the years passed, they reduced the MC bore size - the duals started out at 17 and gradually came down to 15. When I replaced the master on my RS, the parts guy at eurotech (a racer) claimed he used the 14 for better feel and control on his race bike. So that's what I went with and was happy with it - and it didn't scrunch my fingers or pull all the way to the grip.

Just because it came from the factory that way, doesn't necessarily mean it was the best solution!

having bleeding fun... can't get any pressure at at ... will not vacuum bleed, will not pressure bleed upwards.... arrrrggghhhh.,..

doing the rubber band holding brake lever overnight suggestion... tilling cylinder level next
may have to yank back off and bench bleed.

the old grizzled Porsche restorer i apprenticed with (briefly-- he had Issues) would do a bench/vise bleed on a new m/c, then install the cylinder in the car and thread in the hard lines loosely, and do a bleed at the hard line fittings (use a shit-ton of rag/shop towels of course). Useful for automobile-type m/c's (which the /7 BMW's certainly were) having separate steel lines/circuits threaded into the m/c body.

When bleeding new/opened handlebar-mounted m/c's it takes me a combination of sitting overnight and tapping gently with a wrench etc on the line from bottom to top to coax slow-moving bubbles to the top

Bench bleeding is just not necessary. Follow these steps. Bleeders closed. Fill reservoir. Pull the brake lever 1/2" slowly then release. Each time, see bubble come up. Keep doing that until no more bubbles appear. Then tilt the back up and down while working the lever to clear all bubbles. Open a bleeder. Pull the handle just a bit and release. Watch fluid level. Sometimes they will just self bleed at that point. If not, very slowly pull the brake all the way and release slowly. Top up as required. Then do the second bleeder. Rerock the MC to purge that front bubble again. Set brake lever fully and leave overnight. Tap the lines before and after.

A final purge the next morning and you should be looking at a firm brake.

thank YOU!

very timely as I was just getting ready to yank BMC to bench bleed. normally when hydraulic doesn't bleed. my Phoenix brake fluid injector will pump fluid backwards to clear any bubbles. not so in this case. fluid refuses to move upwards, causes quite the mess when pump pressure blows off feeder hose.

followed advice above and now have firm pressure!!!! thank YOU!

front brakes still locked up... release pressure on left caliper bleeder, freewheels nicely.
release pressure in right caliper, no difference.

has to be collapsed left front hydraulic hose.
two hoses added to parts list....