Sorry, a bit out of focus. My '75 R90S was retired to the barn for several years and was sadly neglected. I am resurrecting the machine, so I'll start with the brakes.
The calipers came apart and all was fairly clean. All parts were cleaned up, but one of the pistons has some rust.
It seems that the rust is not internal, not inside the caliper bore, but external to the seal. I think the rust was between the seal and the dust cover. The rust destroyed what appears to be a chrome plating.
Measuring inside the caliper bore, from the the base to the seal is 21 mm. The rust is at 23 mm from the base of the piston, so it seems that would be external to the seal when the piston is extruded under braking pressure.
Big difference if it happened with worn pads, once you put new ones in there "if needed" then the corrosion may get pretty close from the seal. But I have remounted pistons like that once satisfied the corrosion wouldn't be too close from the seal. Some rust converter, a little brake grease inside the dust cover and keep an eye on it later just in case. Make sure the plating is not about to come off farther down, hard to see you really have to try scraping it off with a knife or something. Clean the groove in the cylinder very very well, that is where all them seized calipers problems usually come from. I went and had a look at how much BMW wants for a caliper.....EeeeeK!:eek1
My sentiments, exactly, on the cost of new pistons... yikes! It's not that I'm trying to get buy as cheaply as possible, I want to do a good job, but are those pistons made of gold? I will get new pads so I think you may be right about the rusted area contacting the seal.
Got Picasa? 2-3 minutes from taking the pic and posting. I think you are going straight from your computer....files too big.
Don't worry....I ain't the best with them computers myself but, a pic is still worth a thousand words, saves a lot on typing and some of us only work well when we can see. All in the details....for some of us! Little icon at the top here, square kind of yellowish with a little mountain in it , should say "Insert image" when you put the cursor there. Click on that the box will appear and say "Enter URL of your image". Clisk delete to get rid of the HTTP thing,then paste your "embed image link" from Picasa in there. Picasa: Once you get your pic in there you have to send it(Upload) online then go to "View Online" , click on Links for that image, then select the size you want and at the bottom check "Image only". Then where it says "Embed image"right click on that then "copy paste" that back into the "Insert image" box on here. I tried with the "Image link" on Picasa, did not work, had to do the "Select size" and "Image only" thing. There may be another way with Picasa, but that one is kind of new to me also so I piddled with it last week and it does work. So much faster than Photobucket. Beats asking the nephew.....I think he is still flying from 2 months in India with his mom. We really want to see that bike.....in Old's Cool. Ouch....having a hard time editing but "Picasa Online" should look like what I copy pasted down there. <TABLE style="WIDTH: 100%" id=lhid_tablecontent class=lhcl_contentarea cellSpacing=0 cellPadding=0><TBODY><TR><TD id=lhid_trayhandle class="gphoto-trayhandle gphoto-collapsibletrayhandle"></TD><TD style="VERTICAL-ALIGN: top" id=lhid_tray class=lhcl_tray><TABLE class=lhcl_host><TBODY><TR><TD class=lhcl_column_left></TD><TD class=lhcl_column_right> </TD></TR></TBODY></TABLE> Link to this Photo Link <INPUT id=lhid_link_inputbox class=gphoto-sidebar-inputbox value=https://picasaweb.google.com/lh/photo/2fDSJH6vHllaLL7JoXmyjToYLt4RxkkDiyqZ35FKkQ8?feat=directlink readOnly closure_uid_24m1xq="69"></INPUT> Embed image <INPUT id=lhid_snippet_embed class=gphoto-sidebar-inputbox value='<table style="width:auto;"><tr><td></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From 2011-10-17-1646-31</td></tr></table>' closure_uid_24m1xq="70"> Select size<SELECT id=lhid_snippet_size closure_uid_24m1xq="71"><OPTION selected value=144>Thumbnail 144px</OPTION><OPTION value=288>Small 288px</OPTION><OPTION value=400>Medium-400 400px</OPTION><OPTION value=640>Medium-640 640px</OPTION><OPTION value=800>Large 800px</OPTION></SELECT> <INPUT id=lhid_snippet_checkbox type=checkbox closure_uid_24m1xq="72">Hide album link <INPUT id=lhid_just_image_checkbox type=checkbox closure_uid_24m1xq="73">Image only (no link) </TD></TR></TBODY></TABLE> <TABLE style="WIDTH: 100%" id=lhid_tablecontent class=lhcl_contentarea cellSpacing=0 cellPadding=0><TBODY><TR><TD id=lhid_trayhandle class="gphoto-trayhandle gphoto-collapsibletrayhandle"></TD><TD style="VERTICAL-ALIGN: top" id=lhid_tray class=lhcl_tray><TABLE class=lhcl_host><TBODY><TR><TD class=lhcl_column_left></TD><TD class=lhcl_column_right> </TD></TR></TBODY></TABLE> </INPUT> </TD></TR></TBODY></TABLE> <TABLE style="WIDTH: 100%" id=lhid_tablecontent class=lhcl_contentarea cellSpacing=0 cellPadding=0><TBODY><TR><TD id=lhid_trayhandle class="gphoto-trayhandle gphoto-collapsibletrayhandle"></TD><TD style="VERTICAL-ALIGN: top" id=lhid_tray class=lhcl_tray><TABLE class=lhcl_host><TBODY><TR><TD class=lhcl_column_left></TD><TD class=lhcl_column_right> </TD></TR></TBODY></TABLE> </TD></TR></TBODY></TABLE>
Got it. It worked. I am not a total Luddite, just don't have 12 hr. per day to spend at the computer. Thanks for your help.
This is the rusted piston after some sanding. The plating was weakened and sanded off with the rust. It looks like the defective area could be under the seal, or just exterior to the seal. I guess I'll have to get new pistons.
Time to remove the master cylinder and brake lines. Master cylinder appears in good condition externally. Will disassemble completely, clean and inspect. All the parts appear to be in good condition, just dirty, and the grease is all dried up. Time for cleaning and fresh grease. It is amazing that all the parts are such high quality. These old BMW's are great machines, very well made, and it's easy to work on them. Looks like I will have to buy some expensive pistons, but most of the work is just cleaning and lubrication.
Some stuff I wrote recently to the airlist, might be useful.. Firstly, watch any parts you get particularly the Caliper Dust Seals. I destroyed four Caliper dust seals, all purchased aftermarket – from the U.K. I read everything I could, searched archives, and just could NOT get them on without destroying the seal, particularly on the outer edge where it bonds to the metal stiffening ring. In the end after No.4, I worked out (I'm pretty quick on the uptake, eh?) I had Buckley's of getting one on without destroying it.. In the end I pressed one on using a mates press. Even then it still was slightly damaged. I had to be doing something wrong.... So I got my measuring stuff out. I measured. After I measured some OEM ATE seals I found out that they (aftermarket seals ) are approximately ~0.05mm bigger in diameter! Needless to say the OEM dust seals just pushed in with thumb pressure. The actual fluid seal (aftermarket & OEM) seemed to work fine. But as far as I know you cannot get OEM seals without a piston – and that's how I got mine. The pistons themselves. One was cactus, the other had corrosion in a non- travel part of it's body. In other words it was corroded no where near the fluid side of the piston. So I reused that one. The other was crook, so I stumped up and bought a new one, OEM of course – ouch! Another thing for me was that I stripped the Caliper body bleeder screw thread. I thought about a repair kit you can apparently get, (mate said they were pretty dodgy at beast) but ended up with a good second hand unit. Interesting was that the wrecker had only one 38mm caliper left (mine are 38mm) but heaps of 40mm – so keep that in mind if you need one. Bleeding was relatively easy. I bled holding the caliper up as high as I could with a wedge of timber holding the pads apart. Air came through easily, did the other side, and I got a good lever feel quickly. Thanks to whomever posted that tip on to the Airlist! The brakes themselves are fantastic! Gone is the constant dragging of the pads on the disk that I had before. After 36 years you would reckon that the rubber seal would have had it! Performance seems to me to be much better, I reckon I could lock the front wheel easily now – still not two finger – it ain't a S1000RR either! Finally, I think this job should be high up the list as a thing to do sooner rather that later. Our bikes are getting on and well brakes are Important! HTH OZ
Now would be the time to go with a handlebar mounted MC! I put one one my single ATE front end, and I swear it has more usable stopping power than when I was running the stock ATE dual disks like you are. I used a general fitment Magura MC and it looks good and works great. This being an R90S you might be wanting to keep it all stock, but it's all bolt on stuff, so you could always go back if you wanted, but I bet you never would.
I might consider that master cylinder. I am not going for a show bike or a trailer queen, I want to RIDE this bike, so keeping it all original is not too important. Yes, it will still look like an R90S, but I am going with some basic upgrades - I want it to STOP!
