rear brake not working

I bled my rear brake today. I wonder if I understood correctly your insulation?

I would say that insulating the tube from the brake fluid reservoir to the master cylinder has no effect on the problem. Instead the line from the master cylinder towards the slave cylinder - as well as the master cyl itself - should be insulated. I installed a length of split silicon tube to the unprotected part of the brake line and wrapped the master cyl with alu foil. In fact, it seemed that it might be easier to insulate the top surface of the exhaust pipe with eg. alu tape (don't know if it withstands such high temps, though).

andy1 said:

hi i have a 2008 rsvr factory . my rear brake stopped working as i think its a common fault so i replaced the brake fluid got it working again then i got some heat resistant rubber tube split it in half from top to bottom placed it between the engine and the bottom of the brake tube to the top of the reservoir and after a 1000 miles i still have a good back brake . i hope it helps people with the same problem

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Hi Andy,

Can you perhaps share with us a picture of what you did?

As most of us are aware, a relocation of the MC + Hose + Reservoir will solve this issue permanently.

However it does not help in identifiying the underlaying cause. After reading multiple forums, feedbacks and comments from other RSVR owners, I seem to think that this issue might be caused by cavitation and possibly heat.

My rear brakes still jams from time to time, I am planning to try the below solution out next weekend.

1. Change to a bigger reservoir (to allow more brake fluid storage and also to let the air bubbles come up back form the MC)
2. Hose rubber seals (the ones that are normally used in water taps/pipes) and put it between the MC and the engine. (to recude heat and perhaps minimize the cavitation)
3. Relocating the Brake hose outside, rather than passing right beside the engine. (to recude heat and perhaps minimize the cavitation)
4. Relocating the Banjo head from the current facing upwards to facing downwards and away from the engine. (to reduce heat and also to ensure that no air bubbles from the MC goes to the rear caliper thus the only air bubble escape way is back up to the reservoir)
5. last but not least, full flush and replace with new Dot 5.1 brake fluid.

will keep you guys posted on my test and pictures of what I will do.

if any one have tried this before, do gimme a feedback, thanks

I guess the root cause is pretty obvious. Let's consider how seals are designed. If possible, a seal is designed so that the pressure it attempts to seal presses the seal against a mating surface. The higher the pressure the stronger it presses the seal lip against the cylinder wall.

In the RSV rear brake, the problem arises from large tempereture variations. The brake fluid heats up and dilates while driving. As the fluid cools down and contracts during a standstill, a small vacuum is created inside the fluid system. In that case, the pressure is opposite to the normal operation and the seal will leak and a minute amount of air into the system.

Mikey said:

I guess the root cause is pretty obvious. Let's consider how seals are designed. If possible, a seal is designed so that the pressure it attempts to seal presses the seal against a mating surface. The higher the pressure the stronger it presses the seal lip against the cylinder wall.

In the RSV rear brake, the problem arises from large tempereture variations. The brake fluid heats up and dilates while driving. As the fluid cools down and contracts during a standstill, a small vacuum is created inside the fluid system. In that case, the pressure is opposite to the normal operation and the seal will leak and a minute amount of air into the system.

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True, a good way to see the issue as well, I never thought of it that way before. Thanks for sharing

I was also thinking of filling in a High Temp Brake Fluid which can withstand up to 325°C, but seems there has been a few people trying that without any success thus I think the normal 5.1 brake fluid which should be able to withstand 260°c should be sufficient.

I did a quick heat check on my MC earlier today after getting stuck in bad traffic for about 30 mins. Seems that the MC did not even exceed 150°C. Engine temp was up to 100°C.

Although the master cyl and the hoses are close to the engine casing, the temperature of the engine cover is not that high. My guess is that radiant heat from the exhaust pipe below the brake system could be the one to blame.

If my earlier attemps to rectify the problem fail , I'll just wrap the exhaust pipe in aluminum foil and secure the foil with few steel strings. Aluminum has a very low emissivity so the radiant heat from the exhaust would decrease significantly.