Gen II rear brake - decreased efficiency

[Cosmin]

Hi everyone,

I am writing you with an issue noticed at my '06 Gen II related to rear brake versus trail braking. After previous experience with the CBR1100 Blackbird linked brakes (which I delinked and was satisfied with them - except for increased front dive under heavy front braking plus some increased needed pressure in the lever for activating all front pistons), when I bought the FJR I was determined to delink the brakes but I gave up since the brakes were quite well balanced (trail braking possible until a certain point without activating front) and another reason was that a FJR delink brake kit was not very easy to find (in the absence of a kit I didn't want to start improvising brake lines etc).
Though at the last time technical inspection (the mandatory one necessary for allowing the bike to be used in traffic for another 2 years) the inspector complained about reduced rear brake efficiency. They put each wheel (by turn) in a roller-brake measuring stand and test the brakes using also a pressure sensor either on the pedal or on the lever, depending which brake is tested. He showed me that there is a big gap between pedal force and rear wheel brake effect. This was confirming somehow my suspicions since during trail-braking I had started in the last months to sense some front dive and some 'raising' trend of the bike from leaned position in curves. Another fact was that the last rear pad set is lasting for almost 30.000miles for same riding style and similar roads (compared to max. 20.000 initial 2 sets ) and for same OEM Nissin pads used every time.

I studied more in detail the rear brake system for potential root causes and I concluded that the rear pump is sending pressure splitted to:
- proportioning valve. This one allows pressure to be sent to the rear caliper up to a preset pressure. After that specific pressure some sort of internal valve is blocking/limiting pressure to be further applied to rear piston;
- second split is the metering valve. This one is actually opening at a certain threshold, allowing pressure to be sent also to the front right caliper.

So these 2 valves work in sort of opposite directions: one is blocking pressure to be 'forwarded' further from it and the other one is allowing pressure to pass when a certain pressure is built in the system. Now for my issue I have the following 'suspects':

1) rear master cylinder. It could be that it's not building anymore the needed pressure. Though, taking into account that at higher foot force the front is diving and brake effect is felt (without applying hand lever brake) it's clear that pressure is built in the system by the master cylinder.
2) rear caliper issues. During normal riding (without rear braking) the pads and rotor are not heating at all - so blocked piston issue for lack of rear brake effect is out of question. Also the 2 slider pins of the caliper are fine and lubricated. A relatively inexpensive preventive maintence I could do is to buy the piston and seal rings and test again after that...
3) proportioning valve. A malfunction on this valve could block too early the pressure sent to the caliper. The problem is that it's quite difficult to measure pressures/diagnose and the valve is also sealed (no maintenance possible). Price displayed in US ~100$ (might be 150$ or more with customs tax and transport to Europe - price at local Yamaha is 200Euros plus ).
4) metering valve. A malfunction on this valve could allow too early the pressure to be sent to front, balancing from the beginning rear/front pressures and decreasing the rear brake force. Like 3) above (proportioning valve) it's not easy to diagnose, the price is a little less though. Still I think it's working ok since indeed a higher foot pressure is needed for activating the front (and sense the dive); that higher pressure is needed to reach the metering valve opening threshold I assume.


An option I was evaluating consists in eliminating completely the 'proportioning valve' (remains to be tested if the rear caliper hose can be easily connected directly on the ABS exit where the proportioning valve had the small hose connected from factory). The logical effect would be more rear brake from pedal and less for front; though in this setup the rear ABS could be triggered earlier for what used to be 'medium' brakes (less probable in my case since 99% of the time I ride with wife on bike and luggage and rear is prone to block at light rear weight on bike). Normally the bike should become more friendly while trail--braking (like an un-linked braking system).

Any recommendations would be welcome.
Thanks.

 

[lele]

I never never read so long in my life

is the circuit perfecly bleeded?

 

[wfooshee]

The valves don't work in opposite directions, they work together. The metering valve ensures that only rear brake is applied up to a certain pressure. At that threshold, one of the front calipers is also activated by the rear brake pedal. The proportioning valve is supposed to activate at that same threshold to reduce the rate of pressure increase at the rear brake. It doesn't stop the pressure increase, reduces the rate increase.

Think of a line showing the pressure on the rear pedal graphed against the pressure in the rear caliper. Up to a point, the proportioning valve shows a straight increasing line and the metering valve is a flat 0 line. At the threshold point, the metering valve line turn up and you see pressure at the front caliper. At the same point, the line for the rear caliper still goes up, just at a reduced rate. This is intentional in the design, to force the braking load towards the front as the rear "gets lighter."

The "reduced" efficiency they showed you may be part of that design, but if the behavior of the bike has changed over time, as you feel it has, then I doubt those pieces are the issue. The first thing that comes to mind with "reduce brake efficiency" is air or water in the fluid. Between the ABS and the linked brakes, the bleed process for the FJR rear is significantly different from a standard unlinked system, and although I'm sure you're aware of that, i'd still look into a proper and thorough brake bleed first, which for the rear, includes the lower right front caliper. A complete bleed, including the ABS unit and front brakes, involves a lot of bleed screws and a lot of hydraulic volume.

 

[SunRider]

You might consider fouling of the rear pads/rotor with grease/oil as a possible cause of reduced efficiency.

I see greasy fingerprints on rotors after tire swaps sometimes.

Odds could be increased for this if the shop pulls the wheels too.

Another source of rotor fouling is spray cleaners.

 

[escapefjrtist]

It's already been mentioned above Cosmin, but are you absolutely, positively sure the rear brake has been thoroughly and properly bled? Unless you have a mechanical issue with a component in the rear braking system, GEN IIs, for whatever reason, need more frequent than normal bleeding of the rear hydraulic system.

Is the rear brake lever soft or exhibit more than "normal" travel? If the answer to either of these questions is yes, then begin with properly bleeding the rear system. Figure on flushing 250 - 350ml of brake fluid through the RH front lower caliper + rear caliper. Begin work at the front caliper.

Good luck!

--G

 

[RossKean]

As SunRider mentioned, the pads and/or rotors could be contaminated. They could also be glazed. Try removing the pads and scuffing them up on coarse carbide paper. give the rotors a good clean with a degreaser and scuff them up with carbide paper or a (brown) Scotchbrite pad. You mentioned that the sliders were clean and lubed. Check to make sure that none of the brake lines is crimped or damaged.

If the above plus a thorough brake bleed does nothing, I think the next step is to replace the master and slave brake seals although in the absence of a problem such as leaks or failure to retract properly, I don't think this is the problem.

 

[Cosmin]

Thanks all for replies. Now addressing all comments in arrival order :

- yes, I am pretty sure the circuit is properly bled. Every 2 years I change brake fluid (front circuit, rear circuit, clutch). The method used in the past 4 changes was opening each bleeder (one after the other one is finished), leaving a significant quantity of fluid to leak via a transparent hose into a recipient while monitoring constantly and adding fluid to the proper level in the relevant tank. Even if this method involves some fluid waste (I leave quite some quantity to leak in the recipient and that one I throw away) I am quite sure the change is almost complete and I have no air in the system (proved by good pedal/lever feel after each change, no 'spongy' lever travel etc). Indeed - I mentioned the change is 'almost' complete since I didn't activate (with the test coupler) the front/rear ABS circuit in order to make sure the ABS internal fluid is replaced also but I think that minor quantity will blend with the new fluid and potential water dilutes enough to be harmless.
- thanks for additional details on the 2 valves behaviour, I also noticed the graph in the Service Manual which shows precisely the brake force (rear and front) evolution depending on pedal force. It's encouraging that it's less probable for the 2 valves to be at fault and, based on the explanation, it's clear that my plan of removing the proportioning valve indeed will cause a more frequent ABS intervention for the rear wheel (even for light brakes in the rare situation when I ride alone with no luggage, so very low weight on the rear).
- the rear disc and pads look good; the rear wheel is always installed by me and I am quite careful with all installation steps including making sure no grease reaches the disc/pads (I also use brake cleaner for the disc after installation).

The mandatory technical inspection is due in a few weeks; when I'll visit them I will find out details about the rear brake efficiency (perhaps they will give me the graph). I hope the bike passes without issues the rear brake test, I'll let everyone know.

 

[Fred W]

I vote for air in the rear brake circuit. Seems to happen all the time, especially on 2nd Gens (or later) and also seems to be hard for people to get the air out.

As for deleting the "linked" braking feature in general, I would not be opposed to that idea. There are times when I would like to use the rear brake with no possibility of it applying the front. Maybe I don't really understand what the feature is supposed to provide. I am perfectly capable of applying the front and real brakes simultaneously on my own when I want to. It does seem to create more problems than it solves. Maybe we should start a separate thread to brainstorm ideas on how to do this?

 

[RossKean]

I am perfectly capable of applying the front and real brakes simultaneously on my own when I want to. It does seem to create more problems than it solves. Maybe we should start a separate thread to brainstorm ideas on how to do this?

If you come up with something, it should include having the lower right piston pair activated by the front brake rather than just by the rear pedal as it is currently.

 

[Fred W]

Definitely. Maybe I'll start that thread now.

That thread is here

 

[Cosmin]

Thanks for suggestions, I'll give the rear bleeding another try since it's not that difficult to do and who knows, perhaps I'm lucky to solve it

Regarding 'de-linking' topic - thanks Fred, I already posted some ideas in the relevant page.

 

[kleake]

Did you come up with anything? My 2008 sat for about 2 months without riding. Now upon using the rear brake, I get the normal movement with light braking, but then it is only a light pressure and the lever moves much farther down than it should. I can bottom the lever and the wheel just barely tries to lock. Fixing to try bleeding the system, but I find it odd that sitting for 2 months would have caused an issue like this.

 

[RossKean]

Did you come up with anything? My 2008 sat for about 2 months without riding. Now upon using the rear brake, I get the normal movement with light braking, but then it is only a light pressure and the lever moves much farther down than it should. I can bottom the lever and the wheel just barely tries to lock. Fixing to try bleeding the system, but I find it odd that sitting for 2 months would have caused an issue like this.

Empty the rear brake reservoir using a syringe and refill it with fresh DOT4, bleed the front right lower bleed nipple, bleed the rear caliper. Keep an eye on the reservoir while you are doing it to make sure you don't run it dry. Next do the front brakes and the clutch with more fresh DOT4. Maybe an hour and a half including putting everything back together. It's not uncommon for an air bubble to develop in the off season. It might just work itself out but a fresh fillup of new fluid certainly won't hurt.

 

[kleake]

I had bled the rear caliper before posting that with no change. Just bled the front, 5 minutes worth and got several bubbles. Problem solved, but how does air get in a closed system like that?

 

[Cosmin]

Glad to hear you solved it that fast; in my case I didn't have time yet for a new bleeding but in any case my brake's behavior is much better than yours (the pedal is firm with no obvious symptoms of air in the system).

 

[rvnmaniac]

Having the same issue. Are there 2 bleeding points on the right Front caliper.

 

[escapefjrtist]

Having the same issue. Are there 2 bleeding points on the right Front caliper.

Yes. The upper bleed screw is for the front hydraulic circuit and the lower bleed screw is for the linked / rear circuit.

--G

 

[rvnmaniac]

Okay thanks. time to do some more bleeding this weekend. Odd that i bled the lower front bleed screw but not the upper bleed screw. This was just to put fresh fluid in the system. The front brakes are fine but rear is soft. Seems like it should be the opposite since i did not bleed the upper but did bleed the lower. Everything was fine prior to flushing the system. Did notice the rear master was pretty much dry but the rear break was working normal at the time. Rear brake pads are still usable and rear pistons moved freely after taking the rear caliper off and checking it and the pads.