1980's era 15mm Magura brake master cylinder - My brake adventure continues

[SGTBILL]

Hello again. I have previously posted for advice on caliper identification (mine are Brembo F08 it turns out) as I am renewing the front brakes on my 1978 R100S. A previous owner upgraded to newer Brembo brakes from the original ATE brakes, including a handlebar mounted master cylinder. The photo is my disassembled master cylinder after several bouts of cleaning in my Ultrasonic cleaner (PineSol) and various brushes and picks. My concern relates to the two holes visible in the photo. The left-most hole is larger than the right. There does appear to be a "pin" hole in the right hole but certainly not as large as the left-most hole.

As part of my preparation for the task I reviewed Brook Reams video on the 1983 R100RS which seems to have the same master cylinder.

At about 1 minute into this video the two holes in the MC are visible and both look like the left-most hole in my photo.

I am reluctant to try to enlarge the right-most hole. At least not before asking the Hive whether the holes seem correct? Or should they be similarly sized?

Luckily replacements are available at EME.

 

[stevenrankin]

They are correct. St.

 

[SGTBILL]

Thanks Steven. These holes are for inlet and outlet per Brook's video. That one on the right is very small. does it create (or maintain) some back pressure?

 

[oldcamper]

Thanks Steven. These holes are for inlet and outlet per Brook's video. That one on the right is very small. does it create (or maintain) some back pressure?

If the tiny hole is plugged the brakes will not fully release and the brakes may auto apply as the caliper heats up.

 

[AntonLargiader]

The hole closest to the outlet should be very small. That is the compensating port; it only has to allow fluid to equalize between the brake system and the reservoir when the lever is all the way back. There should be no maintained back pressure.

 

[9842]

Hi
Years ago I replaced a master cylinder (MC) on an RS I had because the front brake would drag. Every two years I would crack the bleeder and a little fluid would come out and brakes would release and wheel would spin.

Over the last few years I had the same thing happen on my 1988 R100RT. last year I changed lines with braided lines and bled. I would bleed them a fews squirts and be done. This year I decided to change the MC because it's almost 40 years old. I remembered how easy it was when I did it before. Call Rusty at Max to get a new Master Cylinder and all parts. A 13 mm MC, crush washers, and dot4. Replace the whole cylinder and connect line. Fill reservoir and bleed. Some say bleed at top bango fitting first, but I didn't have to, I bled the bottom only and have a firm lever. I did have an oops when I left the old rubber gromit in place and it kinda jammed up the release action. That's was ok as I'm used to doing jobs twice and spending extra time on simple tasks. Better off spending time in the garage than house or yard work.

The MC was $250. I'm handy and willing to maintain my machines but Im not a real mechanic. I mention this because you can get a rebuild kit for a lot less money. I didn't chance it because of possible corrosion on the housing and having the rebuild not work out. I'm better off buying the complete assembled MC. I know my limits!

So 2 bolts and a bango fitting, add a little dot4, and bleed. Easy right. Don't forget to trash bag the area around and under the MC, and absorbent rags too. As you know brake fluid will eat paint so be careful.

 

[AntonLargiader]

In case 9842's subtlety is lost on some of you, he's basically telling you what not to do. Don't run a clogged MC for years occasionally bleeding the excess backpressure. DO flush the brakes completely on a regular basis, and hopefully prevent the clogged MC problem in the first place.

Firm lever is generally good, but the lever becomes even firmer when the MC is clogged. Don't be fooled.