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Rear Master cylider on LMs III

Rafael

Rafael

GT Reference
Joined
Feb 16, 2009
Messages
1,096
Location
San Leandro, CA
I'm doing some maintenance on my buddies LMs III. While adding some DOT 4 (as indicated on reservoir MC cap) I noticed that the new fluid separated from the existing fluid. So I read up on this and learned that DOT 5 does not mix with DOT 3 or 4 and figured someone had used DOT 5. So, I decided to flush the system and with new DOT 4. While pumping the foot lever, the piston started getting stuck in the depressed position, so I sprayed a small amount of silicone lube in the area, I made sure it didn't splatter and land in the MC reservoir. It worked for a little bit but then started sticking again and seems to have gotten worse. Any suggestions? Could the seals have been affected by the DOT 5 and are now incompatible with the specified DOT4? Should I go back to DOT 5? Help!

thanks
 
Do not use DOT 5, it is not compatible with Brembo systems. That said, since it is an older bike, make sure the area around the piston is clean and free of debris. If so, it may be rebuild time. Also you should should clean and rebuild the calipers if the system was contaminated with DOT 5 fluid. It wouldn't hurt to check the front brake (handlebar system) as well.
 
thanks John,

Bad news, but at least I know what's going on with the brake system. Fortunately, whom ever used the DOT 5 in the rear did not use it in the front MC. I was wondering if the calipers would be affected, oh welll. Should I split the caliper halves to replace the o-ring between them? Man, I wasn't counting on this amount of work... What about the hoses?, they're the originals.

I recnely bought a new 12mm front Brembo MC for my SP and the cap says "use DOT 3 + 5". I bet it's a typo.

thanks again, Rafael
 
Rafael wrote:
thanks John,

I was wondering if the calipers would be affected, oh welll. Should I split the caliper halves to replace the o-ring between them?
Click to expand...

Yes, you need to split the calipers to replace the o-ring and the seals for the pistons. Complete kits are listed in the parts manual. Lines may be OK, but this would be a good excuse to go with steel braided lines.
 
john zibell wrote:
Do not use DOT 5, it is not compatible with Brembo systems. That said, since it is an older bike, make sure the area around the piston is clean and free of debris. If so, it may be rebuild time. Also you should should clean and rebuild the calipers if the system was contaminated with DOT 5 fluid. It wouldn't hurt to check the front brake (handlebar system) as well.
Click to expand...

John
What is the difference between DOT4 & 5? I was un der the impression that the difference was to do with the temperatue it can handle.
 
Original DOT 5 is silicone based and has the highest temperature tolerance specification. DOT 5 is not compatible with dot 4 designed (Brembo) systems.

Dot 3 and DOT 4 are glycerine based. The difference between 3 and 4 is that 4 can withstand higher temperatures. You can probably search the web to get the specifications for operating temperatures. That said, if you are using Brembos, stick with a DOT 4 fluid.
 
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