1975 r90/6 master cylinder

[clmedlin]

I am rebuilding my r90/6. When I purchased the bike it was unrideable. During the rebuild process, I rebuilt the master cylinder with new brake line, hydraulic line (SS), and all new o-rings and seals, the metal cylinder was cleaned, the piston was cleaned, and the chamber was honed. However, the plastic reservoir still leaks at the connection points to the metal cylinder. I believe the pitting on top of the cylinder is preventing the reservoir from sealing - but I am not positive. Any insight would be appreciated.

 

[20774]

Seems like the rubber o-rings are doing the sealing, so any metal pitting shouldn't come into play, unless there is pitting underneath in the groove for the o-ring. I'm not familiar with that style of master cylinder. I see you have two areas that have o-rings. Do both of those enter the plastic reservoir? Brook Reams has a picture the o-ring placement on his rebuild of a '77 R100RS...it only shows one o-ring needed:

https://www.flickr.com/photos/brook-reams/33344996062/in/album-72157675789624605/

OK, found another one from Brook for the exact same bike as yours:

https://www.flickr.com/photos/brook-reams/26971845311/in/album-72157668453072305/

His master cylinder body is just as pitted as yours. There seems to be a special fixture that clamps both o-rings at the same time:

https://www.flickr.com/photos/brook-reams/26971843001/in/album-72157668453072305/

 

[clmedlin]

Thanks for the input. The earlier 90/6 master cylinders had “2” hole reservoirs. 1 smaller hole allowed the reservoir to attach to the M/C while the larger hole was allowed the piston to draw the brake fluid in to the brake system. I am leaking from 1 or both of those holes. I replaced both o-rings. I have measure the OD,ID and thickness of the o-rings and all are in spec. I have not been able to find a schematic that shows a close up of the locating sleeve and thrust plate to verify I haven’t made a simple mistake in assembly. The big unknown is I had to replace the washer on the bolt - it was missing, and it’s not listed on the BMW parts fiche, so that was a guess. I hope I can solve the leak with out replacing the master cylinder.

 

[ccolwell]

I believe my R90S had the same MC, which I rebuilt (years if not decades ago). I'd suspect the plastic reservoir instead of the pitting.

 

[nrpetersen]

If I'm oriented right, the upper o-ring face sealing surfaces look like they are as-cast. Weird to this hydraulics engr (now retired)! It doesn't simply look like corrosion. What do the face sealing surfaces under the o-rings look like? I can understand how the rings would not be able to ever seal.

You may be able to carefully flat file that surface to reduce the roughness so that the o-rings can seal again. It probably isn't necessary for the flat surfaces to completely clean up.

 

[69zeff65]

Check the plastic reservoir for cracks

If that large diameter washer from inside the reservoir was missing when you pulled it apart there's a good chance the reservoir is cracked or damaged.

 

[collingsbob]

Thanks for the input. The earlier 90/6 master cylinders had “2” hole reservoirs. 1 smaller hole allowed the reservoir to attach to the M/C while the larger hole was allowed the piston to draw the brake fluid in to the brake system. I am leaking from 1 or both of those holes. I replaced both o-rings. I have measure the OD,ID and thickness of the o-rings and all are in spec. I have not been able to find a schematic that shows a close up of the locating sleeve and thrust plate to verify I haven’t made a simple mistake in assembly. The big unknown is I had to replace the washer on the bolt - it was missing, and it’s not listed on the BMW parts fiche, so that was a guess. I hope I can solve the leak with out replacing the master cylinder.

Most washers used to seal oil or brake/clutch fluid lines can be replaced by Dowty washers - commonly used for hydraulic fittings…

 

[clmedlin]

I am very appreciative of all the input. The mention of a washer inside the reservoir is intriguing - there was not one. MaxBMW does not show the breakout of the reservoir in their micro fiche. I have order vitron o-rings to see if they help. I appreciate the comment about dowty washers - could they possibly be used as an o-ring between the reservoir and M/C?

I have resigned my self that if I can’t come to a 100% reliable solution, I will replace the M/C. I want a safe reliable bike and I will abandon the notion keeping stock parts - I’ve already done that elsewhere.

Thank you all. S/F

 

[ccolwell]

If you abandon using model specific standard parts, consider a handlebar mounted MC. While I doubt you'll get much improvement in braking distance, the feel will be considerably better.

 

[clmedlin]

Just wanted to follow up with how this was resolved - there is not a lot of information that I’ve been able to find on the M/C besides the rebuild kits. I inquired about sending the entire unit in to have it professionally rebuilt - but if boring and re-sleeveing the cylinder doesn’t resolve the issue, then the current thought process is to replace the unit. I decided to pull the M/C off and redo my work. I water tested the reservoir to ensure it was not leaking. I sanded all contact points between the reservoir and the M/C to ensure any residue or imperfections were removed. I added an internal o-ring to the thrust plate (this was inside the reservior), and I straightened the plate out some that goes from the mounting bolt to the thrust plate - to create more downward pressure. Lastly, I re honed the cylinder, replaced all the p-rings again and reassembled. The issue appears to be fixed. I am bench testing now, the M/C is pressurized and it is holding fluid, with no signs of leakage.

Thank you all for the input. I’m a new member and this is my first project bike. Lots of learning on my end.

S/F

 

[Omega Man]

Nice.

OM