Clutch "pumps up" and begins to slip when hot only.

[Redcardbob]

Help!
I have an '85 that is having clutch issues. When I bought it recently, the master and slave were leaking, and there was a lot of air in the system, so the clutch wouldn't disengage at all. Since then, I have rebuilt the master and slave cylinders (with Yamaha parts), and replaced the line with a Galfer unit. After this, when the bike got hot, the clutch lever was nice and firm, but once the bike warmed up, the lever would get rock-hard, and the clutch would slip on accel. So, I opened up the clutch to check for burnt parts, but did not see anything unusual. I went ahead and replaced the fibers anyway, scuffed the steels, and installed an HD diaphragm spring. Back together, and the same thing is happening!
Once the bike warms up, pressure builds up in the system to the point that it slips on accel. The lever gets hard, and there is no play before the clutch begins to disengage. If I crack the banjo fitting on the master, I get a small jet of fluid, and the lever gets some play back. I get about 10 minutes of riding before the cycle repeats.
I first suspected the tiny return passage in the master was plugged, but when I pull the lever with the Master Cyl. top off, I do get a small squirt of fluid from the hole. Not a fountain, but enough to shoot a drop over the side.
The lever appears to be the original.

I welcome any ideas!
Thanks, Bob.

 

[Fire-medic]

Yeah, I think you have some debris in the small port of the master cyl. That hole should be capable of a geyser of fluid being displaced, more-than 'a drop over the side.' You might have some hardened accretions/deposits which have broken-loose, and are migrating-around in the hydraulic system. I would remove the line to the slave cyl, and capture the fluid coming-out of the banjo bolt (disconnected) at the other end, as you use compressed air to blow-out the line. Throw a gallon plastic bag over the end of the line, or maybe use a gallon milk container with the banjo bolt end of the clutch hose in it, to capture the discharge.

It sounds like what happens when you have a brake hose go-bad, where there's an internal collapse, and that collapse acts-like a 1-way valve, causing pressure to build and lock-up a wheel's brake, until you crack the bleeder valve to release the pressure.

I cannot think of any scenario with the clutch slave cylinder that would cause something like that, unless the piston was sticking in the bore. You already addressed that.

 

[Redcardbob]

Thank you!
That makes sense, but I wasn't sure of the fluid volume that was considered "normal". Do you think the opening size is critical? Could I drill it out with a tiny bit instead of a piano wire? I don't see a spec on the opening size.
Bob

 

[Fire-medic]

I suspect that diameter is critical, and I don't think you can probably find a drill small-enough to easily do it, without risking breaking it off, because of side-pressure snapping it in-two.

There was a professional musician who was a member here, he used to call-out the exact guitar string note that was needed for this job. I use a pin vise and a single SS strand from a brush I have, to poke-out the hole.

OK I found his post, he called for a 'high-E' string.

Did you bleed to line when you changed it? Accidentally squeeze the lever during the process? You might need to put a guitar string (probably high E) into one of the tiny holes in the bottom of the master cylinder to get some tiny particles out of there. It can become clogged and prevent the fluid from flowing back into the master the way it's supposed to. When the fluid heats up, it builds pressure and partially disengages the clutch (that's how I understand it). You might also want to remove and grease the brass barrel that the M/C diaphragm pin goes into in the lever. This can get just a tiny bit stuck and bind preventing the plunger from fully retracting.

(82) clutch change | Yamaha Star V-Max VMAX Motorcycle Discussion Forum (vmaxforum.net) Post #2.

 

[02GF74]

This is my guess - refer to diagram below.
As the engine warns up, the clutch fluid inside the slave and partway in the hose expands.

The excess volume of fluid would normally be pushed through the vent port but that is blocked. It has nowhere else to go other than to push on piston inside the master, which is up against the circlip so it cannot move and also against the piston inside the slave, hence clutch starts to disengage.

You need to be sure the vent port is clear, I would use a needle or a drill that fits into the hole. I'd recommend removing the internals to ensure you are not poking the rubber seal which may be situated at the end of the hole.

When the piston is out, check there is no scoring or damage to the bore.

DO NOT ENLARGE THIS hole unless you know what you are doing, the engineers at Yamaha (or whatever company make the hydraulics) made it that size for a reason.

Note that there should be some free play in the clutch lever, about 1/2 inch before it gives resistance.

 

[Redcardbob]

Great idea!
Thanks again.
I'll update after i try this.
Bob

 

[one2dmax]

ALSO, do not overestimate the effects of overfilling the reservoir. If it's more then 2/3rds full that can start creating back pressure as the fluid expands from heat. Another issue is with some aftermarket levers that have constant light engagement which can cause excessive heat build up and eventual slipping until the bike is shut down and the system cools down again.

 

[Redcardbob]

ALSO, do not overestimate the effects of overfilling the reservoir. If it's more then 2/3rds full that can start creating back pressure as the fluid expands from heat. Another issue is with some aftermarket levers that have constant light engagement which can cause excessive heat build up and eventual slipping until the bike is shut down and the system cools down again.

Thanks for the info Sean! I did back off on the fluid level a bit.

 

[Redcardbob]

CURE UPDATE:
Thank you everyone for your help! Turns out that the simplest cure was the right one. The return hole was partially plugged. I used a safety pin to open it up, but apparently it was too big to go through. Thinking i had checked that off the diagnostic list, I then began to hit my forehead against the wall looking for more obscure cures.
Thank you to Fire Medic for the "wire brush bristle in a pin vise" idea. The stiff stainless bristle was small enough to go through the small hole, and now The clutch works great!

Thank you for this forum, and for all your help!
Redcardbob

 

[Fire-medic]

It's satisfying when the easy-fix is the answer. In a comment to Sean's 'too-full' mention, I once had a Kawasaki LTD inline-4 I bought used (like nearly-all my bikes) and it had the abominable pull-back handlebars which are part of that package. I was having issues with the front brake, and though I had the sight glass showing brake fluid in the middle range, I found that because of the angle of the master cyl on the pull-back-bars, the fluid level wasn't sufficient to properly-cover the front hole in the master cyl floor. Two things addressed that: one, fill the fluid to the top of the fill-range, to be able to cover that small hole; and two, replace the pull-back handlebars with a pair of SS superbike bend bars, which allowed the master cyl to sit at a better angle, where the holes in the floor were better-covered.

 

[Redcardbob]

Another great point to remember! I replaced the stock bars with a set of flat drag bars, which puts the master cyl. at a slightly different angle. I'll have to think-through the fluid levels to make sure I'm not over, or under-full.
The devil is in the details....

Redcardbob

 

[Traumahawk]

I'm glad that you got everything fixed and that it was a simple solution. Just posting up my issue that I had that wasnt so simple of solution....tohelp others as a possible cause. My 07 had the same issues for years...and it would only happen after a couple of hours of riding. It wasn't fluid level....return hole....or clutch as I had the DD mod. Turns out the hole in the clutch lever that the pin fits into....wasnt as deep as the stock lever. I had after market adjustable levers. With the hole not being as deep, then the pin would always be pushing in on the MC a bit and the clutch would always be actuated a bit, hence the clutch slipping after a couple hrs of riding. The fix was to shorten the pin a couple of MM. If i had a drill press....i couldve drilled the hole deeper in the lever.

 

[Fire-medic]

I'm not sure why you wouldn't notice the lack of free-play for the lever. I was recently working upon the single-opposed piston front brake on a couple of bikes using the SOHC Honda 750/4 front brake, and with the aftermarket levers on the OEM SOHC master cyl, I saw that there was an excessive amount of free-play (just the opposite of what you had). I figured-out what had to be added to reduce the clearance, and I had a friend heli-arc additional material to the lever's contact contour with the master cyl piston. It made a huge difference in the amount of lever travel, especially if you measured it at the ball-end of the brake lever.

I set-up all my brakes and hydraulic clutches to have a minimal amount of free-play. Doesn't Sean Morley have a bearing insert for the levers' pivots on OEM levers which he sells?

 

[Redcardbob]

I'm glad that you got everything fixed and that it was a simple solution. Just posting up my issue that I had that wasnt so simple of solution....tohelp others as a possible cause. My 07 had the same issues for years...and it would only happen after a couple of hours of riding. It wasn't fluid level....return hole....or clutch as I had the DD mod. Turns out the hole in the clutch lever that the pin fits into....wasnt as deep as the stock lever. I had after market adjustable levers. With the hole not being as deep, then the pin would always be pushing in on the MC a bit and the clutch would always be actuated a bit, hence the clutch slipping after a couple hrs of riding. The fix was to shorten the pin a couple of MM. If i had a drill press....i couldve drilled the hole deeper in the lever.

 

[Redcardbob]

Another good point to remember! All of my levers are stock, but that didn't stop me from considering either drilling out the pocket, or shortening the rod. Now that I found the cure, I'm glad I didn't resort to that!

 

[one2dmax]

Doesn't Sean Morley have a bearing insert for the levers' pivots on OEM levers which he sells?

I have some remaining

 

[DreamV4]

CURE UPDATE:
Thank you everyone for your help! Turns out that the simplest cure was the right one. The return hole was partially plugged. I used a safety pin to open it up, but apparently it was too big to go through. Thinking i had checked that off the diagnostic list, I then began to hit my forehead against the wall looking for more obscure cures.
Thank you to Fire Medic for the "wire brush bristle in a pin vise" idea. The stiff stainless bristle was small enough to go through the small hole, and now The clutch works great!

Thank you for this forum, and for all your help!
Redcardbob

You will possibly have same problem soon unless:
1.fluid is clean like water (keep draining and filling)
2.fluid reservoir is clean like new
3.I suggest using a needle, because needle has cone end and will go in until right diameter is reached. If you use some wire, it will most likely have too small diameter to clean hole 100%,

 

[Fire-medic]

You will possibly have same problem soon unless:
1.fluid is clean like water (keep draining and filling)
2.fluid reservoir is clean like new
3.I suggest using a needle, because needle has cone end and will go in until right diameter is reached. If you use some wire, it will most likely have too small diameter to clean hole 100%,

I have one of those millenary kits with several dozen needles of different sizes. None of them fit that tiny orifice. However, my stainless steel brush has hundreds of bristles which are perfect fits for this purpose, I use a pin vise to hold it.

A standard needle/pin like you'd find in a new shirt, is far too-large in diameter for this purpose. Or refer to post #4, if you play guitar.

 

[DreamV4]

I use sewing needle sets, there are many different sizes there, I could always find one, which worked for all holes, including idle jet. I use cigarette lighter to heat up end of a needle to bend, so I can use it to clean 3-4 tiny holes in carb wall. I am not saying needle is right way, I just used them for a long time and it worked fine for me. In worst cases I filed one side of needle end, making it a drill bit to clean hard deposits from idle jets.