3CL Rear master cylinder identification

Freebird

Freebird

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Location
Surrey, UK.
Hi Everyone

I went to do a simple job of changing the brake fluid yesterday which turned into a bit of a saga when the rear one refused to bleed. The master is shot I think, it's drawing air in so it's time for a rebuild. At least it gave me the opportunity to take the swing arm out for a repaint  - it's suffered from brake fluid spillage in the past.

I have used the search function and found a whole host of very useful information regarding rebuilding the master cylinder but am having trouble working out which size it is. I and am pretty sure it's a 15mm jobbie, only nothing in it actually measures 15mm so I wanted to check with you learned lot. The bore is 15.89mm diameter and the spigot that the bell crank acts on is 13.3mm diameter. I read somewhere that this should be 15mm for a 15 cylinder.

Also I have read that Motomecca or "Wolfgang" are good places to get a rebuild kit but the only Wolfgang I can find is a VW Beetle specialist.

Any help as always appreciated.

Cheers

Glen.
 
They are 15mm. Kits are also available from Bevelheaven https://store.bevelheaven.com/brake-related-parts/brembo-15mm-lrd-brake-m%7Cc-kit-fits-round-reservoir-70s-80s-front-rear-mcs/
 
Hi Glen,
They can be difficult to bleed.
Try removing the master cylinder and caliper all in one and try bleeding off the bike.
Make sure caliper nipples are higher then master cylinder.

Andy
 
Yep, 2nd that. I only got mine sorted when I borrowed a vacuum bleeder.
 
KERMIT1200 said:
Hi Glen,
They can be difficult to bleed.
Try removing the master cylinder and caliper all in one and try bleeding off the bike.
Make sure caliper nipples are higher then master cylinder.

Andy
Click to expand...

Thanks for all the info Guys

I did try the above to no avail unfortunately. Odd that the front ones are way below the master cylinder but have no issues.

Glen.
 
Freebird said:
Odd that the front ones are way below the master cylinder but have no issues.
Click to expand...

Hmm .... there are those that would take issue with this statement. See threads on the topic. The splitter on the front fork usually traps air. Consensus is that clamping the lever to the bar overnight usually (not always) helps.
I just bleed as well as I can, then ride. Lever usually pumps up after a few days ???

Ken
 
I had trouble with my rear master cylinder too.

I followed advice from a professional and raised the rear caliper above the master cylinder. I depressed the rear lever and a few tiny bubbles kept rising, after a little pumping and resting while depressed over about a day or so all of a sudden she was bled!!!!!
So:
1). Just keep the bubbles rising.
2). Maybe invest in a vacuum bleeder.
3). Have some patience.

There is just a possibility that the tiny hole in the bore of m/c is blocked, which to my knowledge makes bleeding difficult.
If so, then it needs to be stripped and a bit of fine wire inserted to unblock.
 
SimonR501 said:
I had trouble with my rear master cylinder too.

I followed advice from a professional and raised the rear caliper above the master cylinder. I depressed the rear lever and a few tiny bubbles kept rising, after a little pumping and resting while depressed over about a day or so all of a sudden she was bled!!!!!
So:
1). Just keep the bubbles rising.
2). Maybe invest in a vacuum bleeder.
3). Have some patience.

There is just a possibility that the tiny hole in the bore of m/c is blocked, which to my knowledge makes bleeding difficult.
If so, then it needs to be stripped and a bit of fine wire inserted to unblock.
Click to expand...

Good advise too. The tiny hole you refer to was blocked, but fixing that was not a full cure. I've ordered a seal kit from Motomecca now so will rebuild it all in a week or so.

Glen.
 
I have original Brembo 15mm seal kits in stock... M/Cs are designated PS15, piston diameter is actually 15.8mm.

Remove the entire system from the bike leaving the hose connected, clamp the caliper in a vice or similar so the hose connection is at the highest possible point and hold the M/C so air can travel up the hose and escape out the tiny holes you see at the base of the reservoir.  Fill up with fluid and carefully move the M/C lever and piston only a few millimetres, complete strokes are not necessary.  After a few movements, bubbles will appear as soon as the lever moves the piston only a tiny bit.  Continue working the lever in this manner till the bubbling stops, top up the fluid, screw on the cap and re-install the system.

piet
 
I am exactly the other way
no change there then

first loosen the bleed nipple or nipples, then remove them and grease the threads with copper slip, but be careful not to block the actual bleed channels,
next unbolt the calliper from the swing arm
then fix a clear tube to the nipple(s) and arrange it so that the calliper is hanging as low as possible from the master cylinder, then make sure that the clear pipe(s) have an inverted "U" in their length before terminating in a glass jar, just open the nipple(s) about half or three quarters of a turn and wait for the fluid to climb up the inverted "U", watch the air, when it ceases, without moving the calliper, tighten as best you can the nipple(s), refit to swing arm and test.

CLEM
 
sfcpiet said:
I have original Brembo 15mm seal kits in stock... M/Cs are designated PS15, piston diameter is actually 15.8mm.

Remove the entire system from the bike leaving the hose connected, clamp the caliper in a vice or similar so the hose connection is at the highest possible point and hold the M/C so air can travel up the hose and escape out the tiny holes you see at the base of the reservoir.  piet
Click to expand...

Thanks, and Clem.

The entire systemis off the bike already. Bit confused about how high the master cylinder needs to be though - presumably higher than the caliper amd virtually horizontal if air is to escape out the tiny holes?

Glen.
 
The caliper needs to be higher than the master cylinder.To allow the air to easily travel out through the nipples.
You should be able to fit the front brake lever(if std),to beable to pump the fluid.
Depress lever,undo one of the nipples, allow fluid and air to escape do up nipple ,release lever -repeat.

Andy

 
Freebird said:
Thanks, and Clem.

The entire systemis off the bike already. Bit confused about how high the master cylinder needs to be though - presumably higher than the caliper amd virtually horizontal if air is to escape out the tiny holes?

Glen.
Click to expand...

Yep, you got it... just needs to be above the caliper, air will always rise.

Just need to act slowly and cautiously, any nervousness will have you spilling brake fluid all over the place. :D

I normally fill brake systems with a vacuum device, drawing fluid from the M/C at the caliper bleed nipples.  Quite often, the brake remains spongy, usually because an air bubble is trapped at the hose connection to the reservoir.  The vacuum simply draws fluid past the bubble wihout being able to dislodge the bubble itself.  Trying to pump fluid through the system with the lever is often futile, not enough fluid is moved by a single stroke to shift large bubbles as a whole, they simply move back and fro with a little fluid flowing past them.  This can be continued for hours without any noticable pressure being achieved.  By introducing only a small amount of fluid into the system, it slowly replaces the air contained in the bubble, eventually all the air will be expelled.

Dumbfounded a Ducati Pantah crew at Spa a few years back with my method.  They had been mucking around with their rear brake for about 3 hours, I had it bled in about 8minutes, including removal and refitting of the system. 

piet
 
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