Rear brake problem

[Fishermark]

I noticed the pads on the rear brakes were getting worn, so I bought new ones and installed them. Pretty straight forward - didn't have any problems. Bled the rear and front brakes and flushed the brake fluid with new DOT 4 till it ran clear. No bubbles.

Everything is fine till the test drive. Had nice firm rear brake pedal... until I didn't. It went from hard to all the way down with no stopping power. Pumped the pedal and had some brakes again. Went home and re-bled everything front and back. Ran almost a whole bottle of new clean brake fluid through. Another test drive. Worked great. Till later that afternoon. Again - sometimes the brake pedal would go all the eay down and other times it would be rock hard and work well.

I've never seen brakes work inconsistently before. Thoughts? I plan on rebuilding the master cylinder, but it seems mighty coincidental that it only began this issue after changing out the pads. (I even bought and used genuine Honda brake pads).

One thing I did, which might have caused a problem is I simply pressed the pistons into the caliper when installing the new pads. I have since read that a better way to do it is to release the pressure at the bleeder valve when pushing th piston back in. Maybe the back pressure caused an issue with the seals in the master? But the inconsistency has me puzzled.

 

[mikesim]

My "guess" is that you still have air in the system somewhere. Let it sit overnight and rebleed the brakes. Before doing so, take a plastic handle screwdriver and tap each caliper gently which can loosen any captive air bubbles. Good luck

Mike

 

[ARKNT]

My guess also is that there is still air in it. You said you flushed the front fluid, are the fronts working normally? Sometimes it is a bit tricky getting all the air out.
These are linked brakes so be sure to take that into account when bleeding.
And of course be careful. An unexpected loss of brakes can be a real eye opener.

Arknt

 

[seagrass]

Did you bleed the two brake nipples on the front left caliper (sitting on the bike)?

The second nipple is for the third piston (middle piston) and is linked with the rear brake caliper.

Seagrass

 

[Fishermark]

I did in fact bleed the front and rear brakes, and I am aware that the front left caliper is linked to the rear. It has two bleeder valves and I bled it in conjunction with the rear. I would normally agree that there is air in the system... but whenever I have had issues with brakes - if there is air in the system then it shows all the time.

I don't have the service manual for this bike. I need to buy a copy. I know there is also a proportioning valve in the system. Is there anything special you need to do with that when bleeding?

 

[DirtFlier]

I've never messed with the proportioning valve and don't think it has any external adjustments. I had my bike since new in Dec 2009 and have bled the brakes about every 2 years since 2011 without any problems.

 

[harryhendo]

Pressing the pistons back into the caliper, slowly and carefully, is normal and shouldn't have anything to do with the lack of braking power. I usually loosen the cover of the master cylinder and put a rag underneath it to catch any fluid overflow that might occur when the caliper pistons are pressed to their fully seated position.

 

[DirtFlier]

Pressing the pistons back in during a brake pad replacement is somewhat similar to bleeding your brakes because any air is pushed back towards the reservoir. I always drain the reservoir during a pad replacement to avoid any spillage of brake fluid when the pistons are pushed back.

 

[Fishermark]

Well, it's curious. I went ahead and ordered a rebuild kit for the master cylinder. Will go ahead and rebuild the master and see what happens.

 

[ARKNT]

I don't recall needing to do anything special with the proportioning valve when bleeding. I believe the front piston gets fed directly and the rear caliper is fed from the proportioning valve. It would be another spot for air to maybe get trapped. Basically the proportioning valve has a piston and a spring in it to give the rear a proportion of the total pressure.
Good luck.

Arknt

 

[Coyote Chris]

Well, this is a good one.....we have a 100 percent functional bike being ridden on a regular basis. The rear pads are changed, putting the rear pistons in a different operating position in their cylinders and the fluid is flushed. The brakes are bleed one time and fail like there is air or a defective master cylinder. Then the brakes are bleed again and they are intermittant. At this time, there is no evidence of leaking and the level in the reservoir does not change.

Has anyone seen an intermittant master cylinder?

It does sound like there is air in the system but I have never seen that lead to an intermittant issue. Not saying it cant happen. With the absence of leaks and fluid level changes, I guess I would rebuild the master cylinder also.

 

[Fishermark]

While waiting for the master rebuild kit to arrive I put the bike up on the center stand and knelt down to look at the master cylinder - and how to remove it. While knelling there I push down on the brake pedal with my hand - it goes all the way down through its travel like there is no fluid or resistance at all. Push that a couple of times, (not trying to pump it up or anything), then next time I push the pedal it stops and locks the brakes like normal. Very hard pedal again. It is so strange! It is all or nothing. I will post what I find when I remove and inspect the master cylinder.

 

[Hondafan]

You may want to purchase one of these. You connect it to the bleeder and draw the fluid out through the caliper. I use one and it works perfectly.

https://www.amazon.com/Performance-Tool-W87030-Automotive-Bleeder/dp/B013VUZ7WG/ref=psdc_15706941_t1_B0002SQYUA?th=1

 

[rc_heli_nut]

You may want to purchase one of these. You connect it to the bleeder and draw the fluid out through the caliper. I use one and it works perfectly.

https://www.amazon.com/Performance-Tool-W87030-Automotive-Bleeder/dp/B013VUZ7WG/ref=psdc_15706941_t1_B0002SQYUA?th=1

I'll second that, using one of these vacuum bleeders is how I bled brakes on my NT, except I use a small 12v electric vacuum pump instead of the hand pump. These type of vacuum reservoirs are easier to use with a continuous vacuum source in my opinion, you can open the bleeder just right to get the flow rate you need. Just make sure to keep that master cylinder topped off as you go. When connecting vinyl hose to bleeder screw, it's wise to make a vertical run with it, several inches up from the bleeder, then to the catch reservoir, this stops air from going back into the caliper. Keep the catch reservoir, vinyl hose, all higher in elevation than the bleeder, cable tie all this to the bike if you need to.

On this bike, the highest spot is the steel line running along the frame to the LF caliper, if there's air trapped there it could take a lot of fluid flushing to get it out. The symptom Fishermark describes is typical of air still in system. I would try bleeding the system again before rebuilding the master cylinder, rear caliper first, then LF caliper second. Open only one bleeder at a time.

 

[Fishermark]

I've got nothing else to do today while waiting for my kit. I will try bleeding again more thoroughly. I do have a vacuum pump kit. I will try that and see.

 

[ST1100Y]

I went for a bigger, air powered one... makes the job a breeze, priceless...

 

[Fishermark]

Bled the brakes again today. Front and back. Went through a whole bottle of DOT 4. Everything is clear, no bubbles at all. Test drive again. Works perfect for the first half dozen stops, and then no brakes at all. Here's a video - sorry the sound is a little muffled.

 

[Coyote Chris]

Bled the brakes again today. Front and back. Went through a whole bottle of DOT 4. Everything is clear, no bubbles at all. Test drive again. Works perfect for the first half dozen stops, and then no brakes at all. Here's a video - sorry the sound is a little muffled.

Just to be crystal clear....it firms up, you go riding....and roughly stop 7 you get nothing...like a bad master cylinder seal. Then, like in the video, it decides to firm up suddenly...if the bike were on the center stand and you would nail the brake 6 times over five minutes while not moving, would it do the same thing? I am gonna be real surprised if this is air in the system. I am gonna guess that when it is working, you can make a stop using just the foot peddle and the rear disc gets warm and the corresponding front disk also gets warm. I have to go with an intermittant rear master cylinder....they do happen....see this.... https://www.postandcourier.com/automotive/car-talk-bad-master-cylinder-could-be-causing-brake-pedal-to-stop-working-without-warning/article_bd0d446f-8462-55cd-b2a1-44c9668ff3f3.html

 

[Fishermark]

You summarized the situation 100% correct. That's exactly what happens.

 

[Coyote Chris]

You summarized the situation 100% correct. That's exactly what happens.

As a mechanic, I dont believe in coincidences...maybe the Master cylinder was about to go anyway...maybe doing the original bleeding put it over the edge...maybe somehow (reaching out there) some dirt or ? got under a seal or ?. Sure would be interesting to carefully disassemble it and see what there is to be seen. Once, on my way to Reno from Spokane, I was on my trusty 1983 bare gold wing and it had linked brakes....suddenly I had no front brake....the lever just went to the handlebar. I pulled into the first Honda dealer and he had a front Master cylinder rebuild kit and put it in for me then and there....it was a good feeling to have two brake systems but I am here to tell you if I had to depend on my VFR's rear brake or my VStrom's rear brake to get me home, I would be dead....its like the bike speeds UP when I try them alone....