Intermittent Soft Brakes

At the end of my commute on Friday, I noticed that the front brakes felt very soft/spongy. They were working, but not providing nearly the stopping power they usually do. I had to brake really early for stop signs and couldn't get the bike to stop quickly without a lot more rear brake than usual.
I took the bike out on Saturday to see how they felt and was very tentative with the brakes. Everything felt fine and I went to a parking lot and did several quick stops with no problems. Since my commute is on the highway, I hit the highway and the brakes were still no problem. On Sunday, the brakes were just like normal for the 200 mile ride.
My commute is 35 miles, but I turned it into 60 since Friday was so nice. Around 25 of the miles are on the highway and there was very little stop and go traffic. I didn't notice the brakes being weird until about the last 5 miles, which is why I headed for home. The only difference I can think of is that on Friday, the bike sat in the sun at about 85 degrees but it was in the garage until I rode it on Saturday and the bike was on the move all day Sunday.

Any ideas?

 

My mechanic said something about repeated use causing heat, which can cause the fluid to boil and produce air. More heat & more air = less stopping power (the softness you're feeling?). Try bleeding the brakes and see if that helps. New lines might be next if that doesn't help (assuming the pads are fine).

 

If you havn't got braided lines, they can get spongey with age & heat won't help.

 

Are the braided lines a drop in replacement or is there more to it than that?

 

Bolt-on replacement; just gotta get them bled completely.

Stock Yamaha brakes DO "fade" quite a bit when they get good and hot; at their best they're still a bit spongy. Braided stainless steel lines will cure that. I've only replaced the lower line so far on my 550 Seca (the upper will need to be custom made because of the low bars) and the improvement in feel was dramatic.

At the very least it sounds like you need to flush out all your old fluid with an "extended bleed" and ensure they're bled properly. ANY air will contribute to heat-related fade.

 

A related question, how often should the braided stainless lines be replaced? I assume they still need to be replaced since they have the rubber under the braiding don't they?

mikeg

 

I changed out all my lines. Not hard, but do a thorough job of bleeding. The brake fluid itself is not likely to boil, but if any moisture (water) has accumulated in the fluid, it might on heating, thus adding to the spongy feel. The original lines swell and allow for spongy feel. The Braided have a powerfull fell and very good feed back as there is nowhere for the fluid to go. The old lines should go anyway (26+ years is a long time on a wear part!) and the difference in price is minimal. IMHO

 

I need to change out the fluid, so I will just do the braided lines at the same time. I will also be changing the pads while I am working on the brakes. Any other maintenance I should look into while I am in brake mode?

 

Clean/rebuild the MC and Calipers while your at it. Would hate to have done all the other work, just to have one of those fail. Don't forget to pull your rear wheel and check on the rear brake pads. They delaminate with age and you may need new ones.