Motorcycle Front Disc Brake Problem

Hi Chris,

Welcome to CR4.

Going on the assumption that your brake system is hydraulic, the only thing that comes to mind is air trapped somewhere in the system when you re-installed and bled the caliper.

However, if there was air trapped in the system, I would think that you would probably feel the sponginess in the brake.

I cannot think of any other element in the brake system between the handlebar mounted reservoir (assumed) and the caliper that would expand due to the heat.

When you rebuilt the caliper did you use Yamaha approved parts and procedures?

Are you sure you bled all the air out?

Does the brake feel firm or spongy when applied?

Something obviously changed since the rebuild.

Regards - KJK

That's my take too.

GA

Somet5hing is preventing the liquid returning to the reservoir.

Hydraulic fluid may be incompressible, but it isn't immune to thermal expansion and contraction.

I know nothing about your bike, but I'll bet there's a check valve, or bleed valve that is either clogged or stuck. The brakes should release, with minimal drag.

I think entrapped air would have already put you in the ICU.

Check the plumbing and valves. Bleed the system, just in case.

Hey dj.

Not trying to be snarky. But we don't ever want air in a hydraulic brake system........................failure would be imminent.

Edit}}}Oops............you said reservoir. My bad. You may be right. It shouldn't be filled past the line on the side.

I would also park this bike until I had it fixed.

Hey there Mark, hope you and your family have been able to keep your feet dry. Anyhow, I've seen similar problems with closed hydraulic systems that were over filled and not having enough room for thermal expansion. And being the brake pucks are the only thing that will move, they'll apply the brake. I'm surprised that he hadn't noticed his front brake being touchy or being catapulted over the handle bars! I, too, would park it until the problem is solved! DJ

moisture in the brake fuid will act this way but only if the fuid is above the boiling point of water.

Hi Dj95401

If this was the case, I would have to look at the breather on the main cylinder

Cheers

Joe

There might also might be moisture in the system.....heat basically forcing the water to vaporize and pressurize the system. The fact it has seized indicates moisture was in it...and perhaps not completely eliminated.

Correct piston rebound, enough to give a small pad-to-disk clearance when you release your brake, is normally forced only by rubber sealing ring's elasticity. But for this to happen, the piston itself should be able to slide freely (if rubber ring was not there). Obviously this is not your case, and since you don't mention any leak, sealing ring is likely (?) to be correctly positioned. What's left to blame is ruined piston and/or cylinder surfaces, possibly from an outer rubber boot rupture (common for unorthodox practices during pads change) and dust, debris and humidity inserting where it shouldn't, and causing this damage. This condition was to be addressed at your previous rebuilt. Now, I would do it again, after lightly honing the bore and verifying surface condition allows rebuilt. Safety and brake's wear because of this behavior alone, justifies the cost. S.M.

Great info everyone. Thank you so much!

The bike is parked and I will pull the entire system apart again.

While I'm at it I think I'll replace the brake line too considering it is 28 years old.

I did inspect the cylinder and piston for damage and didn't see anything, but I will look more closely on the next rebuild.

I will report back in a couple of weeks with what I find, so it's all on record for others who might experience the same problem.

Thanks again,

Chris

Check the Master cylinder as well as the slave and brakeline.

GA

I was about to make that point but you beat me to it!

Somebody suggested a check valve in the circuit, but if there were one the brakes would stay on once applied, not a good idea.

Codey

It is not uncommon to have check-valves in brake circuits (aka 'Residual Pressure Valves'), there is one in my wife's old Beetle. They are usually located in the master cylinder. They usually allow a small amount of backflow to relieve the main pressure, whilst maintaining enough residual pressure to ensure pedal travel is not excessive. I'm not sure that this arrangement would be required on a motorcycle, and even if it were I'm not sure this would be the cause of the problem in this instance.

That's right, there are sometimes valves in the circuit, eg in the original Mini there was a valve to limit pressure to the rear brakes, to try to prevent wheel lock. I just meant there couldn't be a check valve to completely stop reverse flow or there'd be a problem.

I think Envelope Guy may have hit on the answer. Many years ago I worked on a Land Rover with adjustable master cylinder pushrod. The pedal travel had increased and some misguided soul tried to cure it by extending the pushrod. Result - brakes binding.

Codey

Many cars have a pressure reducer for the rear brakes. Most are linked in some manner, to the ride height of the rear suspension, the heavier the load, the lower the rear end, the more pressure is sent to the rear brakes....but this valve does not prevent fluid returning once the foot is off the pedal.....

It just prevents lightly loaded rear wheels from locking up.....

You can read more here:-

http://www.familycar.com/brakes.htm

Look for:- Proportioning valve or Equalizer Valve

Hi Holzfeller, the 'check valves' are actually proportioning valves, that limits the amount of pressure applied to the rear brakes, to help prevent them from locking up during heavy braking. Codemaster is correct #22, there is no check valves in automotive braking systems.

Hi dj95401. What I am referring to is not the same as the proportioning valves or pressure reducing valves you often see. It is a check valve, in fact, to be precise, it is a double or two-way check valve, although VW refer to it as a 'Residual Pressure Valve'. Its' purpose is to maintain a residual pressure in the brake circuit when the pedal is released in order to avoid excessive pedal travel when the brakes are re-applied. Here is a diagram:

As you can see, if the main valve was seized the pressure in the brake circuit would not be relieved.

If I'm corrrect, as I also own a Yamaha motorcycle, the master cylinder has an adjustment screw for the lever that contacts it's piston. If that screw is turned too far in, it pushes the piston past the point where it will allow the fluid to go back into the reservoir. Check that as well.

I have another idea. Get rid of the Yamaha altogether. I had a 1981 Special 650. Once parts started failing when the bike was about 6 months old, they never stopped. I finally sold it for scrap. I'll never have another Yamaha motorcycle!

Hi There,

I ride motorcycles and do all of my own maintenance barakes and all. Firstly when you rebuilt your caliper.. Did you take out the caliper seals? Did you clean out the seal grooves? There is nearly always a build up of corrosion (I live in the uk so get road salt corroding the seal grooves) or dirt( Brake dust etc from dry environments) under those seals . If you did not cleans them up then just fitting in new seals was a waste of time and money. Also did you put any grease on the seals to get the piston back in? If so bad move.. grease swells the rubber on brake seals. Use a little bit of brake fuild to lubricate the pistion and seals and it should pop in (Provided you have cleaned out the seal grooves)

However if this only happens on hot days then it may be down to fluid expansion .. as i have lived in africa with very hot temperatres with bikes with disc brakes i never experienced this problem.

Maybe a good idea to strip out the caliper and clean everything, also replace all the fluid with new and rebleed it all. Maybe worth the effort to also replace the rubber brake line with a new braided brake line ,many brands available out there.

It may also be worth the time and effort to strip your master cylinder and clean that out and fit a new kit in there too. Some of the master cylinders also seem to get a blockage when the fluid is very old and seems to "crystalise or get gummy particles form) and the ports get gummed up. And what maybe happening is that as the temperatures rise, the fluid may expand enough to block a port from returning the fluid bake to the reservoir.

Good luck.

Hi Folks,i have had a fair amount of experience on these and i would go for dirt in the master cylinder or the clearance being wrong on the at rest setting as RWC suggests ,the master cylinder piston only has to restrict the return orifice slightly and the system will "pump up" and i suspect is made worse by some of the temperatures you guys get.Check the clearance between the Screw end and the piston and set it to just above factory minimum(wider gap)if you have the information or @ half a turn back (remember to re tighten lock nut)and see if re occurs.Don't forget to get used to where the brake starts to bite before riding,it won't be a lot different but enough,this should cure it as long as the caliper hasn't re siezed.Ride Safe T

Check to see if you have a vent hole that is plugged in the master cylinder. Change fluid out and bleed using DOT 4 fluid

i'ld check the relief valve in the master cylinder. make sure the brake "puck" is not stuck in the cyinder. try pushing it back in with a c-clamp. check for corrosion in the cylinder. comments are also good advise.

sorry guys, it's already been said.