Master Cylinder Rebuilding

[RB_RT]

Greetings all

I have recently purchased a 1983 BMW R100RS.
The bike looks very nice, but suffers from neglect.
My short list of to do items are getting the engine to run better, bypassing the Pulse Air system and getting the brakes to work.

I have two sets of questions for the group. I have checked the compression and I am getting 110 on the left cylinder and 105 on the right (sitting on the bike).
This seems low to me. I am going to do a leak down test in a few days to see what that can tell me. What should I expect for compression on this year and model? How well does this model respond to higher compression pistons?

Secondly, the brakes were good enough to hold the bike still on a slight incline, but not functional for riding. The front brake lever would come back to the grip and the rear went all the way down. I took apart the calipers and cleaned them, then flushed the system. The front brake came back but still feels soft. The rear still goes to the bottom. There was no leakage from either cylinder. After removing both cylinders, I took them apart. The bore of the front cylinder is clean, with no pitting or distortion according to my gauge. The rear is clean as well. Here is my question. Should I consider the new Master Cylinder with reservoir or just go with the rebuild kit for the front?
Thanks for any feedback

Roy

 

[20774]

Roy -

Welcome to the forum!

I'm not sure where your RS falls, but the early models were 9.5:1 compression whereas the later models were 8.2:1. I'm thinking yours is the lower compression. You asked how it responds to higher compression pistons? Are you saying you have them or are you considering changing to them. I suspect that going to higher compression pistons are probably going to little bang for the buck. Your valves are probably the smaller valves so they can only take in so much air/fuel. I believe you would need different heads.

I think your compression should be in the 130s. If you did the test cold...and if you did not physically raise the slides in the carbs...you will have lower values. You can't just open the throttle since the CV carbs respond only to air passing through the carb throat...the throttle is only connected to a butterfly plate after the carb.

As for the master cylinders, you seem to have a leak somewhere and since there's nothing on the outside, it could be that the seals are just not up to the task anymore, so I'd replace them if you in this far. Also, consider replacing the brake lines. If they are stock/original to the bike, they have broken down inside and balloon up under pressure and that could contribute to what you're feeling.

 

[lkchris]

Doesn't seem like low compression to me.

Fairly easy to fit BMW 9.5 pistons, just be sure to match the size code stamped on your cylinders. (Most are B) Only nikasil-spec pistons in nikasil cylinders.

When switching to 9.5 pistons, the only jetting change required is one notch higher on needle jet ... according to Bing.

BTDT

 

[stevenrankin]

list

Kurt is spot of with how to do the compression test, first I would double check your results lifting or removing the carbs so as to get the proper air flow.

As far as bumping the compression up. Your bike would have the 8.2 to 1 compression. Changing to the 9.5 to 1 will add a little bit of power. I did this on one of my bikes and did not have to change jetting or needle position. Nor would you have to change the valves or modify the heads. The tricky part will be getting pistons and such to match up and play nice together.

I myself would not attempt to put new higher compression pistons along with new rings in an old cylinder regardless if it is nickasail or not. Despite the supposed benefits of greater longevity and less wear of nickasil cylinders, I have found them to in my experience no more reliable or better wearing than the old cast iron linings. Nickasil cannot be rebored and unless I were absolutely certain the old cylinders are in perfect shape, I would not go to the trouble of installing new pistons and rings without reboring. This kind of thing requires a lot of bore measurements and understanding of how the cylinder is shaped and what the tolerance are. In my case I had bores that looked great for half of the bore, the other half was out of tolerance resulting in the fact if I had just put in new pistons and rings, I would have had problems with sealing later down the road.

What I would recommend if you are set on raising the compression is to go with one of the aftermarket kits now available at Ted Porter's, Boxer 2 Valve, or a few other companies. To be honest, even if you just want to rebuild, I would suggest this method. The kit comes with new nickasil cylinders, 9.5 to 1 pistons rings. the overall price is not bad compared to BMW. I would have done this if such a kit had existed when it was time to rebuild my bike. Instead, I had the nickasil coating bored out and the cylinders sleeved with a cast iron sleeve. Then, I used the 9.5 to 1 pistons. My understanding is there are also places who will bore out the nickasil, re coat, with nikasial old cylinders. What you choose to do is up to you. My method chosen was the cheap way and has lasted just as long as the original system lasted. The kit way would be the way I will go if I ever have to rebuild again.

So, forgive me for writing a book about the engine rebuild. I will now plow into a discourse on brakes for your bike.

I will start with the rear brake. There have been reams of discussions regarding the proper bleeding of the rear disc brake system on the bike of your year. Again, Kurt makes a very good point, as long as you have things apart rebuild with new rings and bits. At this long after the bike was new, I would change the brake lines as a matter of course. They are after all 37 years old, the original lines don't last forever and if the bike has been neglected, one of the first things to go are the brakes and the lines. To be honest if the bore of the master cylinder is good, a rebuild kit is all you should need. NOW the bear of a problem with the rear Brembo brake is it's position, how it is mounted on the bike. There is a very good chance due to the way the cylinder is located, for an air bubble to get trapped in the caliper and no amount of pumping or bleeding will dislodge it. The solution is to remove it from it's mounts, position it so the bleeder valve is at the top. Place a spacer or something between the pads and bleed it that way. I would check Brook Ream's website or recent thread on a restore of a 83 RT, Having seen Brook's work, I can almost bet he has a video or detailed pictures of what I am talking about.

For the front brake system, I would again ditch the old lines and replace. IF the bore looks good, a rebuild will suffice. At least the front brake system is easier to bleed and shouldn't take too long. Again I reference to Brook's threads, or his website.

I hope this helps and is not overwhelming. St.

 

[PAS]

This spring I replaced all the brake lines on my 81 RT. I went to the local Tractor Supply and bought a large syringe and the proper tubing to tightly fit on the end. For the rear disc I emptied the reservoir, unbolted the caliper and let let it hand so the line had a straight shot up to caliper. I filled the syringe and let the bubbles rise to top and expelled them then I pushed the fluid up to the reservoir. Best bleed I ever did and fast! Brake pedal was nice and firm! Basically did the same for the fronts.

 

[ccolwell]

In the past, I rebuilt master cylinders and calipers on my R100 and R90S. In each case, the rebuild worked and lasted. Do pay attention to the position of seals and springs as you disassemble an MC; how they fit back in is not intuitive (IIRC). Definitely replace the brake lines. I had good experiences with steel lines in the front (I never much used rear brakes pre ABS). Lastly, should you decide to reseal the calipers get some red grease for reassembly.

 

[stevenrankin]

Stainless brake lines

I did install stainless or steel brake lines. There was a slightly noticeable improvement. I won't go on record stating this improvement was because I switched to steel or because I installed new lines because the old ones were getting old. I will say, I didn't have many miles on the bike at the time and so I think, the improvement was do to going form flexible lines to steel. St.

 

[ccolwell]

I did install stainless or steel brake lines. There was a slightly noticeable improvement. I won't go on record stating this improvement was because I switched to steel or because I installed new lines because the old ones were getting old. I will say, I didn't have many miles on the bike at the time and so I think, the improvement was do to going form flexible lines to steel. St.

I have put them on a fairly young (4 yr old) airhead. They did make a noticeable improvement.

 

[stevenrankin]

Yes

For the cost, I would say "yes", stainless brake lines are worth the change out. I have them on both bikes. My RS has the dreaded disc brake on the back. To be honest, I cannot fathom why BMW changed from drum to disc. I got nearly 190K of wear on my drum brake, it always worked 100%, other than periodic adjustment it required no maintenance. I could lock up the back wheel anytime. The bloody disc brake, on the other hand sucks, switching between my R80RT and the RS, the rear brake on the RS never performs like it should. Pads wear out too fast due to the dirt and abrasives kicked up onto the disk. And the I have to mess with changing fluid and bleeding to boot.

If I ever come across a 77RS with the drum brake for a parts bike, or the parts to convert cheaply, the disc will go.

Please don't send me a bunch of posts trying to convince me of the error of my ways on this. I have been riding the RS for a long time and I keep my bikes in tip top shape so there are no hidden problems with the disc system. I just plain hate it. If I didn't have the R80RT with the drum to compare too, I probably would not be so adamant. St.

 

[RB_RT]

Thanks for the feedback. I appreciate it. When I checked the compression the first time, I propped the CV Slides open. After reading the feedback, I removed the carburetors and did the compression test again. This time both sides came up to 120. Mission accomplished there.
I have ordered new rebuild kits for the front and rear Master Cylinders and will also replace the brake lines.
I use either a MityVac bleeding pump, or a vacuum bleeder that attaches to the air compressor to bleed the brakes first, then bleed them manually to verify the pedal or level is firm.