Master cylinder

[Cliff A.]

1984 R65ls Original owner, low mileage.
First noticed short throw on front brake lever 7or8 years ago started changing fluid regularly (probably to late).
Stored last 3or4 years, (health and persional issues). Much better now.
Recently went out operated front brake, pistion went in, stayed in.

Any tips on MC rebuild/Parts?
Brake lines?
Rebuild calipers?
What else should I address where it has sat so long? THANKS

[montmil]

At the least, the MC needs a good cleaning.  Any brake fluid in the MC reservoir?

[Cliff A.]

It was empty, I filled it no change.

THANKS for interest

[georgesgiralt]

Hello !
The MC is easy to open and inspect.
If the bore are not corrosion pitted, you can buy for relatively cheap an repair kit (it is a NEW piston with rings already fitted in). do not forget to clean BOTH bores  from the bowl to the cylinder. One is so tiny it is often overlooked.
So you can repair the MC easily (I do hope the bore is good because the whole MC is expensive and it could be difficult to sleeve it : in France law prohibit it for safety reasons).
As you look at the MC, look at the caliper also. if Brembo the repair kit is not so expensive and make them new again.
Do not overlook the brake lines. They have an expiration date .... and steel braided lines are not that expensive.
Front brake is often your sole life insurance on the road...

[Bob_Roller]

If there is not too  much corrosion in the master cylinder bore, it can be cleaned up with Scotchbrite pad on a hardwood dowl put into a drill motor.

[Cliff A.]

Took all your advice, cleaned up pretty good, not mirror shinny but seems smooth, can't feel any pits with toothpick a few discolored spots.

If you all see any issues let me know,
Thank Thanks Thanks

[mrclubike]

If you get a rebuild kit make sure you order it by the size.
do not rely on what the parts book says.
If you flip the cyl over you should see the size cast to the bottom.
Mine was supposed to have a 12mm (for single rotor) but  someone put a 16mm on it.
So I had to replace even  if  it was still good.
The singles came with 12 or 13mm a double could have  15 or 16mm.
I would use a 12 or 15mm. If I  was going to replacing it .
I am using a 12mm on my single.

Same goes for the Brembo caliper .
You will need to take it apart and  measure the piston diameter.
If you are lucky some one may have changed  it to the standard 38 mm instead of the stupid 36mm special diameter caliper BMW put on them.

[georgesgiralt]

I think the 36 mm Brembo  were put to mimic exactly the ATE look-a-like they used before. Changing the piston size may have involved a new certification of the bike. Whis was/is costly. So keep everything equal except the colour of the darn thing.

[Cliff A.]

The MC does have a 15 cast into its bottom and I also measured it.
The bike is original (original owner) next step lines and Brembo calipers

Thanks all

[mrclubike]

I think the 36 mm Brembo  were put to mimic exactly the ATE look-a-like they used before. Changing the piston size may have involved a new certification of the bike. Whis was/is costly. So keep everything equal except the colour of the darn thing.

The standard 38mm Brembo FO8 caliper  is a direct replacement to the very expensive 36mm Brembo and the NLA ATE caliper.
It works perfectly. I have over 10K miles on mine with a 12mm master cyl.
This is exact one  I have on mine. From this supplier
http://www.mgcycle.com/index.php?main_page=product_info&products_id=1034
if you have duels you will want this one for the others side.
http://www.mgcycle.com/index.php?main_page=product_info&cPath=110_115&products_id=834
The OEM 36mm Brembo F08 is $460.00
The 38mm Brembo F08 is $114.00 and it comes with new brake pads.

[nhmaf]

Along with cleaning the bore, AND the reservoir, you'll also have to make sure the TINY orifice that allows fluid to flow back from the piston bore into the reservoir (it will be a small hole at the bottom of a dip or well in the reservoir and is only a fraction of a millimeter in diameter.   That hole can get plugged with hardened brake fluid and mess things up as well.  Clean it CAREFULLY with a small piece of wire or some wooden toothpicks - you don't want to booger up that little hole.

[Cliff A.]

the price on those calipers is amazing!
I cleaned the little reservoir hole.

parts are ordered

any opinions on Dot 4 verses 5.1 brake fluid. All parts will be new or well cleaned

[Bob_Roller]

DOT 5 fluid may not be compatible with the rubber seals .

[Tony Smith]

DOT 5 fluid may not be compatible with the rubber seals .

In fact BMW (and Snowbum) say that they are not.

[Barry]

Cliff was talking about Dot 5.1

It's confusing that the bottles are labelled up synthetic but 5.1 is glycol based and compatible with Dot 4 and seals.  The problem with the DOT categories is they specify performance levels rather than fluid chemistry so 5.1 is a valid designation in DOT's eyes but it confuses the hell out of consumers by making them think it's similar to DOT 5 silicone based fluid.

I've used it and it was fine although when I suggested that on the Airheads list I got my head bitten off by Messrs Bum and Cutter without any explanation of why they objected which is their want on occasions.  Regardless  it's officially compatible with Dot 4.

It turns out that although 5.1 does have a higher performance level in terms of boiling point than Dot 4, it's lower in viscosity for use with anti lock braking systems and it's possible there can be some silicone content.  And eventually if you dig deep enough there is a suggestion that 5.1 trades higher performance for a shorter working life in terms of moisture absorption. In other words it's more hygroscopic. That shouldn't be a problem with annual fluid changes but in the end I reverted to DOT 4 on the basis that the extra performance of 5.1 wasn't  needed and a good DOT 4 will last longer.

Bottom line is you can use 5.1 but don't mention it on the airheads list.