'93 ZXR750L Clutch (again)

I got sick of throwing empty stubbies at the bike and decided to have another go at fixing it.
The problem is clutch slip and have had this problem since I bought the bike.
Starts when bike gets hot - just a little bit of slip at higher revs which gets progressively worse over the next 10km's or so until it barely takes off from a standing start.
I've tried new OEM and Barnett frictions, steels and springs and a mix of both with no luck.
The problem (as I see it ) seems to be the slave cylinder not releasing properly, ie the piston not returning back into the slave cylinder properly and keeping too much pressure on the push rod and hence holding clutch pack open.
I've tried pulling apart slave several times, cleaned it all, replaced piston seal and internal spring. The bore and piston is good. The push rod is straight.
When the clutch line is attached to the slave (and slave is assembled) I cannot push the piston in at all. If I loosen the bleeder the piston will go in (with gentle help from a g clamp)
When the clutch line is off I can easily and smoothly push the piston in.
Everything else about the clutch works fine. Gears select smoothly etc.
Sorry about the long post but trying to supply as much info as possible so "the think tank" can come up with some ideas. HELLLLLLP

Next bit back up the line is the clutch master cylinder, have you checked it out yet?

Next bit back up the line is the clutch master cylinder, have you checked it out yet?

Nah Mick, I guess that's the next thing.
Problem is I'm not sure if I could diagnose (sp?) the problem by pulling it apart and looking at it.
I'm presuming that the piston in the slave should be able to be pushed back in??
My manual is packed away somewhere amongst 2,000 odd magazines in storage so I guess I better go find it and try and find an exploded diagram of the master.
Thanks for the help

Well I haven't got a lot of experience with hydraulic clutches, but it seemed like the next logical step to me.

logical

Back off pal... there won't be none of that shit going on here


Well I stripped the master cylinder down (found an exploded view on the net ) but couldn't see anything wrong. Everything seems to be in the correct order and good condition.
I understand the basics of how the hydraulic system should work. You squeeze the lever. The piston in the master cyl forces the fluid through the line and hence against the slave cylinder piston, and when you release the lever the clutch springs should be able to push the piston in the slave back again and hence the fluid back up the line to the master.
But If a g clamp won't push the slave piston back in against the fluid, there's no hope of the clutch springs doing it and I can't see anything in the master that wouldn't allow that to happen
I'll wait for some more ideas.

the clutch springs don't push the slave cylinder piston back in via the pushrod - the hydraulics actually 'suck' it back into the cylinder bore. You generally won't be able to push the piston back into the cylinder if the whole system is full of fluid, properly bled and sealed.....there is nowhere for this pressure to go. That is why it only works when you loosen off the bleed nipple.

If the clutch works properly when the bike is cold, it will be working properly when the bike is hot. I don't think the problem is with the slave cylinder/piston assembly at all. But it is a tad hard to diagnose over the Intraweb...

What sort of engine oil are you using ? Has this been changed when you change friction plates and/or steel plates ?

the clutch springs don't push the slave cylinder piston back in via the pushrod - the hydraulics actually 'suck' it back into the cylinder bore.

Yeah that makes sense. I was starting to think along those lines last night whilst lying in bed. Holding your thumb over the end of the master you can feel it suck your thumb back in when you release the lever
The bit I don't understand is that after clutch starts slipping, I can pull off the slave and push the piston back in (after releasing bleed nipple) and there will be no slippage for 100km or so. (if I don't do this and take it for a ride the next day the clutch will still slip like it did the day before.)
I'll order some new seals for the master cylinder just in case. (it may not be sucking back hard enough??)
I'm using mineral based bike oil and have put new oil in every time I try new plates or steels.

But it is a tad hard to diagnose over the Intraweb...

Oh, I don't know. You seem to be making more sense than I do, and I'm sitting close enough to kick the bike
I'll let you know what transpires
Thanks for the help, much appreciated.
Simon

Did you check the both the ports in the master cylinder body ? Haven't read this thread thoroughly, but it seems to me that if the slave isn't returning properly even with the assistance of a g-clamp, then something is blocking the return of fluid.

Just a thought.

Cheers,
Brian

the clutch springs don't push the slave cylinder piston back in via the pushrod - the hydraulics actually 'suck' it back into the cylinder bore. You generally won't be able to push the piston back into the cylinder if the whole system is full of fluid, properly bled and sealed.....there is nowhere for this pressure to go. That is why it only works when you loosen off the bleed nipple.

Gos,

I don't think that's quite right, mate. I'd have thought that apart from the spring in the slave cylinder (presumably designed to take up any slack in the system and keep a tiny bit of pressure on the pushrod), the hydraulics are no different to a brake system. As we know, it's easy to push a brake piston back into the caliper, the fluid simply flows back into the reservoir and raises the level (essentially the opposite of what happens when brake pads wear and the piston moves further out of the caliper body).

Cheers,
Brian

Gos,

I don't think that's quite right, mate. I'd have thought that apart from the spring in the slave cylinder (presumably designed to take up any slack in the system and keep a tiny bit of pressure on the pushrod), the hydraulics are no different to a brake system. As we know, it's easy to push a brake piston back into the caliper, the fluid simply flows back into the reservoir and raises the level (essentially the opposite of what happens when brake pads wear and the piston moves further out of the caliper body).

Cheers,
Brian

gidday Brian
Yeah that was where I got my original thought from.

I don't understand what you mean by "both ports" in the master? There's only one hole in the master bore (apart from the ends of course) and that's the hole that feeds from the seperate reservoir.
I know what your thinking though and that was one of my original ideas that there was some type of return hole that was blocked and not letting the fluid to return into the master. Or am I missing something?

Appreciate all the ideas
thanks
Simon

.... I'll order some new seals for the master cylinder just in case. (it may not be sucking back hard enough??)......

thats a good start. It will remove this as a possible cause, and going through a process of elimination is the only way to get to the bottom of shitty problems like this one.......keep doing the hard yards, you'll get there...It might also be worth talking to a local brake shope, to see if they can't do a very light hone of the slace cylinder, with piston and seals removed. It can't hurt to get another opinion, and you never know, just a couple of light passes with the right size honing stone might make all the difference ?

....I don't think that's quite right, mate. I'd have thought that apart from the spring in the slave cylinder (presumably designed to take up any slack in the system and keep a tiny bit of pressure on the pushrod), the hydraulics are no different to a brake system. As we know, it's easy to push a brake piston back into the caliper, the fluid simply flows back into the reservoir and raises the level (essentially the opposite of what happens when brake pads wear and the piston moves further out of the caliper body)......

I'm hearin' you mate - the little spring behind the piston does take up any 'slack', against the pushrod. What you are saying about pushing the piston back in, and the fluid level rising in the m/cyl is dead right - but, if the whole system is full, bled properly, and sealed, then you won't be able to squeeze the piston into the slave cylinder, as there is nowhere for the fluid to escape to ? But, most 'sealed' hydraulic systems are rarely filled to max capacity......so you would think that there would be a bit of room for some excess fluid....??

I recently did a hydraulic clutch for Bogan - he had the opposite problem - no clutch action at all basically - you could pull the lever in a couple of times, then it went 'rock hard', and no clutch release at all ??? It was weird...........until he bought a new (stock) pushrod.......when we compared the new to the old, the new one was about 12mm longer !!! - I had to push the slave piston back into the cylinder with a small g-clamp (after soaking some of the fluid out of the m/cyl), to install properly. After that, the clutch worked just fine.

I am not a big fan of hydraulic clutches.......good old cables rule !! Even the MFP12 has a cable, can't beat it really...

I am not a big fan of hydraulic clutches.......

Oddly enough, neither am I at the moment
The master kit should turn up tommorrow so will get it going this weekend and let everyone know what happens.
Thanks again
Regards Simon

I don't understand what you mean by "both ports" in the master? There's only one hole in the master bore (apart from the ends of course) and that's the hole that feeds from the seperate reservoir.
I know what your thinking though and that was one of my original ideas that there was some type of return hole that was blocked and not letting the fluid to return into the master. Or am I missing something?

Appreciate all the ideas
thanks
Simon

Gidday Simon,

No worries mate. You had me scratching my head wondering if my memory was really going to pieces, so I went out to the garage and looked at an old ZXR master cylinder body. If you remove the plastic inlet from the reservoir, you'll find there are two holes that allow fluid to move between the bore and the reservoir ; the smaller of these is the return port. I don't recall exactly how this works, but the bottom line is to make sure both ports are completely clear.

Cheers,
Brian

I'm hearin' you mate - the little spring behind the piston does take up any 'slack', against the pushrod. What you are saying about pushing the piston back in, and the fluid level rising in the m/cyl is dead right - but, if the whole system is full, bled properly, and sealed, then you won't be able to squeeze the piston into the slave cylinder, as there is nowhere for the fluid to escape to ? But, most 'sealed' hydraulic systems are rarely filled to max capacity......so you would think that there would be a bit of room for some excess fluid....??

I recently did a hydraulic clutch for Bogan - he had the opposite problem - no clutch action at all basically - you could pull the lever in a couple of times, then it went 'rock hard', and no clutch release at all ??? It was weird...........until he bought a new (stock) pushrod.......when we compared the new to the old, the new one was about 12mm longer !!! - I had to push the slave piston back into the cylinder with a small g-clamp (after soaking some of the fluid out of the m/cyl), to install properly. After that, the clutch worked just fine.

I am not a big fan of hydraulic clutches.......good old cables rule !! Even the MFP12 has a cable, can't beat it really...

Good thinking 99, I hadn’t considered what the impact would be of leaving the reservoir cap on. I’d imagine there is a tiny bit of breathing afforded even with a closed cap, but I’d also imagine that it would take a fair old bit of pressure to get a piston moving in those circumstances.

Agree completely on hydraulic vs cable, over the years I had a couple of different cable arrangements installed on the old ZXR, and both were far superior to the weight and feel of the OEM hydraulic setup – from where I sit an occasional adjustment and regular lubing is a small price to pay for the vastly improved feel

Cheers,
Brian

If you remove the plastic inlet from the reservoir, you'll find there are two holes that allow fluid to move between the bore and the reservoir ; the smaller of these is the return port. I don't recall exactly how this works, but the bottom line is to make sure both ports are completely clear.

Cheers,
Brian

Ahhhhhhh. I think we may be onto something here. I only found one
Will check master body again tomorrow and try and find that elusive hole.
Thanks and regards
Simon