Spongy brakes

I’ve recently seen a few threads about soft brake pedal/lever issues including
https://www.fjrforum.com/forum//index.php/topic/170524-bad-abs-hydraulic-unitmodule-2007a-per-yamaha-dealer/page-1
and
https://www.fjrforum.com/forum//index.php/topic/170958-new-sponginess-travel-to-rr-brakes/

My issue wasn’t precisely the same but something here may be useful to somebody with spongy brakes.

My front brake got mushy, without the system having been opened. Disassembly found that the rubber in the mast cyl had become soft and swollen. It’s hard to tell in the pics of the pushrod assembly but obvious in the pics of the cap diaphragm.







I’ve seen this happen in brake systems when petroleum products such as power steering fluid or transmission fluid have been added to the brake system. And I’ve seen all manner of fluid snafu in 30+ years of making a living working on cars. Some people have no business opening a hood. Nothing but DOT4 has been used in my bike, most recently


So , a mast cyl kit and new cap diaphragm from Yamaha, and YAMAHA fluid, and all is good as new.

A few bleeding observations:

I use an air powered vacuum bleeder. It draws the fluid through fast enough to suck the air downstream. Still, the path is convoluted, considering the linked system and ABS module.

On hydraulic systems where the bleeder is lower than the fluid source (most bike and car brake systems and especially hydraulic clutch systems) it’s sometimes helpful to push fluid UP from the bottom to the top, since air wants to travel up anyway. There are a couple ways to do this. Before any attempt to push fluid “up” you’ll want to have flushed the system with clean fluid already. Trash settles at the lowest point- the calipers. That’s why when a good mechanic does a brake job on a disc system he’ll open the bleeder, and preferably apply vacuum, while pushing the caliper pistons back into the caliper bores. Just flip up the calipers and push the pistons in and you push the crap that has settled to the bottom back up through the ABS modulator and master cylinder. I’ve seen a lot of service advisors call a customer back after a brake job and tell them that they now have to buy a master cylinder. I’ve also seen the service department “eat” a lot of master cylinder replacements, when the customer was no dummy.

The slickest way to bleed uphill is with pressure bleeding equipment-expensive and nearly obsolete. If bleeding with vacuum, speedbleeders or the normal “pump it up, hold it, and open the bleeder” hasn’t worked I have solved the problem by pushing the air “up”, by pushing in a caliper piston or slave cylinder pushrod-AFTER flushing clean fluid through the entire system.

I also like to “exercise” the ABS system. A car/bike can go years without the parts in the ABS modulator moving if the brakes never approach lockup. Some newer generations of ABS systems cycle the ABS solenoids at every startup. Activating the system helps keep parts moving and can help free a stubborn air bubble. My driveway is gravel so keeping my ABS parts moving isn’t a problem.

Exercise your ABS, or do anything I mention here AT YOUR OWN RISK! Just because it’s on the web doesn’t mean it’s smart or safe.

It won’t hurt my feelings if mods merge this post with another thread.

"and open the bleeder” hasn’t worked I have solved the problem by pushing the air “up”, by pushing in a caliper piston or slave cylinder pushrod-AFTER flushing clean fluid through the entire system."

My only comment here is be very careful doing this with an FJR ABS system!

There are very tiny ports in the ABS block which will become extremely difficult to unblock if contaminated.

It is not possible to back-flush them because there are check valves in parallel which will allow fluid to flow freely around the blockage.

That's very strange rubber cap and seals swollen like that. Like you said it happens when oil is in system, sure looks like that happened.???

My 05 master and clutch caps looks great yet. Serviced every 2 years. DOT 4 fresh fluid always.

Maybe the brake fluid you bought was ????. Never know . How would you know? Brake fluid, oil mixed can't see any issues.

Thanks for the discussion. The rear brake pedal travel to engage the brakes is well over 1" and the rear brake doesn't have a lot of effect. I've put off bleading the rear brake again all of last summer but I will be taking care of that and the rest of the system in the next week or so if the rain holds off.

DennisJ said:

Thanks for the discussion. The rear brake pedal travel to engage the brakes is well over 1" and the rear brake doesn't have a lot of effect. I've put off bleading the rear brake again all of last summer but I will be taking care of that and the rest of the system in the next week or so if the rain holds off.

Click to expand...

Typical behavior when the RF caliper needs to be bled. New fluid and a full bleed will bring things back to normal.

--G

Good advice on cycling the ABS, and SpeedBleeders. Two items I would like to add here:

When bleeding brakes, if you remove pressure from the brake lever slowly, you will not move much fluid in the system. It is much more effective on a brake bleeding job if you release (side-step) the lever to allow it to snap-return after each compression, as fast as the return springs can pull the lever. Then the fluid you moved during the compression of the lever stays moved, rather than fading back into where it was when you started. You really want to replace the old brake fluid with clean new fluid, not merely to bleed air from the brake system.

You can replace the highest banjo bolts in the brake systems with bleeder banjo bolts, to avoid the problems of chasing air bubbles down in a vertical hose. Any air at the highest banjo bolt will then be easy to bleed off. Always replace the hydraulic washers with new ones when removing banjo bolts; do not re-use the old hydraulic washers, if you want to avoid leaks. Bleeder banjo bolts each cost about US$15~$20 at the right shops, and maybe US$45+ each at the wrong shops. Shop around.