Hydraulic Clutch Fluid Pressure?

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supernova2, Feb 9, 2:04am
Anyone know what sort of pressure a car clutch hydraulic master cylinder produces in normal operation!
sr2, Feb 9, 2:36am
I couldn't give you an exact figure but for example a 7/8 inch clutch slave with a .6 square inch pistonwould need 1.66 psi for every pound of pressure it puts on the clutch fork. My guess would be well under 100 psi, nothing near the 1500 to 2500 psi max in a braking system.
supernova2, Feb 9, 3:12am
That sounds logical.I guess in most cases a clutch pedal hits the floor/stop rather than maxing the pressure.I've got an odd situation here with a hydraulic clutch which obtains it's fluid through about 1.5mtrs of steel pipe from a common (with brakes) tank.Disconnect the pipe at (well at least close to) the master cyl and nothing flows out, nor will it pump out at the slave cyl.Pipe is downhill all the way from tankPut a bit of air into the pipe and it bubbles in the tank.Disconecting at tank is a PIA to say least.From the look of the installation the master cyl was fitted to the bulkhead almost before the car shell was assembled.OEM is the only master cylinder available world wide so that indicates to me that master cyl is not a common replacement item.

To complicate it a bit more its a csc system so if i put fluid into the csc pipe with a syringe it builds up a palpable pressure and then drops off ending with a bit of a"pissssssssssssssssss&quo-
t; sound.No drips from bottom of bellhousing.The csc is available from multiple aftermarket sources.

So what I'm thinking of trying is to connect the comp air to the inlet of the master, wind up the pressure slowly and see what happens at the csc.

As you can see I'm trying to convince myself that i dont need to drop the box.Also need to find out why the master part of the system wont drip or flow when the downstream pipe is disconnected.

As the bits all need to come from o'seas (and expensive) want to make sure that I only order what is needed.Its been here for what feels like forever and I've been putting it off but got to get it sorted.

I know by now you are all saying "what the f is this thing!'Well its a 2001 Renault Megane Scenic RX4(AWD).

Any suggestions (apart from fire bomb it) welcome.
xs1100, Feb 9, 7:41am
could look at get it rebuilt possibily have had it doen through autostop
snoopy221, Feb 9, 8:23am
First suggestion is to pressure bleed it backwards from the slave cyl nipple.
That way when the master cyl uncovers it's compensating port and *sucks* a tad it's sucking FLUID and not a liwwle wee bubble of air in the line.
supernova2, Feb 9, 10:37am
Can't as no nipple on slave cyl.From hunting the net it appears that getting the system to work is next to impossible.Appears that the idea is that the air will eventually find its way to the top.The bit that has me stuffed is why air will go up the line from master to tank but fluid wont go the other way.The bits of pipe that can be seen as like new so doubt they have beendamaged in any way.
supernova2, Feb 9, 10:40am
Was thinking of that but from what i can see of the master cylinder its a sealed plastic/resin tube.But first I need to work out if there is actually anything wrong with the master cylinder.A new master is $800NZ
skin1235, Feb 9, 10:45am
any pressure at the clutch master port when you disconnect the pipe!
same pressure at the slave if you reconnect the master and disconnect the slave!

( trying to prove the master is actually working)

what sensation do you get on the pedal!
skin1235, Feb 9, 10:48am
swollen primary seal on master cutting off the fluid inlet port or crap in that line and floats up to release air but blocks port when attempting to flow the other way!
skin1235, Feb 9, 10:54am
most clutch slaves are self bleeding, back to the clutch master and then to the reservoir,
your issue is not bleeding but pressurising the slave, right!
look hard at the master - it has to be able to flow fluid both ways - until it can it will not work
sr2, Feb 9, 11:06am
I have to agree completely with Snoops, (as per bloody usual ya drunken bastard mate!). You can still bleed the slave from the flare fitting; just back of the tube nut (where the pipe goes into the slave) and find the point where half a turn seals and unseals it and bleed as you would with a conventional bleed screw.
If you can raise enough pressure to get the clutch to disengage (i.e. you??
supernova2, Feb 9, 11:54am
Can't disconnect pipe at outlet from master as it is between pedal box and firewall and looks like a dash out to get to it.no pressure at slave connection.
pedal feels almost normal but i think that is just the return springs
swolen seals/crap- could well be.However I can blow air up the line from master to tank with no resistence.tank clear plastic and fluid spotless.Logic would say that if i leave line disconected from master fluid will drip out but nothing.
the slave is a concentricie part of the release bearing and inside the bell housing.No flare nuts just a clip on/off line fitting (bit like an injector elec plug).

Might try applying a vacumm to the line at the slave connection and see if i can drag fluid through or pressurise the tank

I think it has a suffed slave hence the pissssssssssssss sound and perhaps someone before me has blocked off the feed line to prevent the brake tank draining away down the clutch line.Is it not possible that a clutch system will work for some time without a resivoir as in effect the fluid just goes back and forward between master and slave !

Will have a further look at it sunday.
skin1235, Feb 10, 7:23pm
does the clutch not work currently!
if not is it just the master starvation!
consider a remote reservoir for the clutch alone - it is a non pressure pipe ( apart from gravity ) almost anyjerry built bottle will work
disconnect that feed line to the master ( cut it if need be at a convenient location and join another from a bottle with a tight sleeve ie piece of fuel line 50mm long)

yet you say the pedal feels acceptable ( but could be the spring tensions )
so assume the slave is working
that spring tension is (again I assume) the pressure plate closing again - so the slave is operating in some fashion

what is the actual problem
thejazzpianoma, Feb 10, 9:14pm
This price sounds silly, as does the notion that only OEM is available. Not having a go at you here just saying whoever is telling you this maybe feeding you a line.

Can you confirm for me you are after Renault part 7700431845 !

If so that should be available without too much drama direct from the U.K for say just over $100 + Shipping.

It should be available as sachs part 6284 600 657
and Valeo part 804645

Don't be too put off if say Valeo are no longer producing it as usually these things hang around as old stock from online suppliers etc.

If it was me in your shoes (and I am not sure on the legality of this) if I didn't have any luck following the manufacturers bleed instructions I would have a sit down with the customer.

Explain that hourly rates are expensive and that its likely either the slave or master cylinder, the testing you have been able to do without removing the transmission indicates its more likely to be a master cylinder issue. Explain that a master cylinder is $800 from the NZ supplier who has to order from overseas.

I would offer to directly import the part (for whatever, say $250) but explain that it's not going to be viable to return it if it does not fix the problem.

Basically, I would just clearly explain where I am at, the time frame involved and what your options are. Then its up to them to decide what they want you to do.

I have not looked into bleeding instructions, known issues etc for the RX4 Clutch, obviously you want to research those too.

Hope that helps, let me know if you need more help with the parts.
thejazzpianoma, Feb 10, 9:28pm
BTW, looking at the master cylinder picture, it appears to be one of these plastic jobbies that in my experience are known to warp, crack and generally cause trouble with age.

If you the problem you are experiencing is needing to pump the pedal a few times to get some pressure then I would say Master Cylinder is the most likely cause, especially as you say there are no drips from the bell housing.

However. fill us in on what the symptoms actually are. Sorry if I am repeating some stuff, I haven't read all the replies above (lazy bugger I know).
bill-robinson, Feb 10, 10:44pm
does the clutch actually work at all when you push the pedal down!
or does the pedal go to the floor!
supernova2, Feb 11, 8:25am
Didn't get to it on Sunday.

The OEM parts are Valeo

Anyway the Master Cyl part is 7700431845 (Valeo 804645)with nz price of $362.49+gst.The slave is 7700107635 (Valeo 804509) with nz price of $816.85+.
Sorry got the prices back to front.
Nothing i have tried will make the clutch release and yes the pedal goes to the floor but silly as this might sound it "feels" right but i dont know how strong the pedal return spring is.When I say feels right it has a bit of freeplay and then loads up.

The funny thing about all this is there is no fluid at the supply line to the Master Cyl.As that is just a unpressurised line I might just use a bit of plastic tube to bypass and see what happens.If i can get fluid to the Master cylinder then if nothing comes out the bellhousing end of things then i know I have to dismantle the car to get the master cyl out for inspection.

I think the slave is stuffed as if I apply pressure to the line at the bellhousing the pressure leaks away and clutch wont release.(It looks as if the csc fitted is not a correct part as the lines are about 30mm away from the locating slots in the bellhousing casting)To complecate this the cross refs show 2 different csc's with different coupling diameters - the correct size is 12.2mm.Aftermarket csc is available in NZ

Jazz where do you think I can find these bits online!

So far I've spent a lot of thinkling but not much doing so not much chargable so far.
skin1235, Feb 11, 6:13pm
"Nothing i have tried will make the clutch release and yes the pedal goes to the floor but silly as this might sound it "feels" right but i dont know how strong the pedal return spring is. When I say feels right it has a bit of freeplay and then loads up. "

you could also have a frozen plate - rusted to the flywheel or pressure plate - even when the clutch pressure plate opens the clutch plate is not released from the engine revs

the detail of pressure leaks away would surely say it's going somewhere yet you say no sign of fluid underneath so how robust is that method of pressurizing

any way to visually check if the pressure plate is actually moving, an inspection cover perhaps

what was the original reason for it to be in the wrkshop
skin1235, Feb 11, 6:17pm
reach in with your left arm and press the pedal - you'll soon know if the pedal feels about right cos your arm will only just be strong enough to depress the pedal - unless you're hulk hogan masquerading here as supernova - and holding it down without bracing your elbow against the seat will only be a few seconds before it falls off ( your arm, lol, you know what I mean)

if by arm you can slap it down and hold it there it is not working correctly, it is not opening the pressure plate
bill-robinson, Feb 11, 6:17pm
it is possble that you have an over centred clutch, and a slave cylinder that is out of travel. as skin suggests a visual check of clutch in operation is needed.
supernova2, Feb 11, 9:16pm
I think we are all off track now.There is no fluid either side of the master cylinder so much as the suggestions are appreciated fairly unlikly that slave has overextended.Only a small view point inside bellhousing but given the "no fluid" situation there is nothing operating.

In shop as "clutch wont release"

Have injected fluid into csc pipe connection then applied 100psi onto csc pipe connection.Holds for a few seconds then drops away slowly with a psssssssssssssssssssssssssound.Have repeated process and same every time.No sign of leak anywhere but must be unless csc holds about a litre of fluid.

I'm told that it was looked at elsewhere and quoted $3-4g to fix but there is no sign of any actual work been done.No spanner marks, no hand prints, usual dirt and grime undisturbed.The only "unusual" thing is the fluid in the tank is absolutely spotless as is the inside of the tank.

I think that the csc has failed and drained the tank (which is common with brakes).The previous "worker" has somehow blocked the pipe between the tank and master.

Much as I don't want to I think it's got to be out with the box to replace the csc.Given the NZ price of the csc that quote of $3-4g is perhaps not far away from reality.I mean who else puts the inboard cv joint inside the gearbox case and uses a cv boot that bolts to the case and has a bearing at the shaft end!.Whats the bet the boot is a squillion $$ as well!Haven't had a good look yet but apparently on some of these RX4s the sump is part of the bell housing.WTF!

All I hope is that whoever designed this thing never gets his hand on designing for Nissan!
thejazzpianoma, Feb 11, 9:21pm
There is a reason I don't have much to do with the Frenchies.

BTW, I like the sound of your method with the compressor and your reasoning makes sense to me. Silly question (not really on the ball this morning) when you do that the air is staying behind the fluid isn't it! Its not somehow bubbling past the fluid causing the release of pressure and the noise!

I am just thinking if you connect directly to the slave cylinder but from below (like on a hoist) the air might bubble past.

So long as its staying behind the fluidI can't fault your method.
thejazzpianoma, Feb 11, 9:29pm
Bump. sorry edited the above so just bumping in case you miss it.
supernova2, Feb 11, 9:59pm
Thanks Jazz.I failed Frenchat school.Now I think that the French are failing me!I can see why local garages don't want Euro work after they strike something like this.

It's not the fault but the inacessable design that causes the problems.I do think part of the design is a result of changing something that was made to be left hook into right hook.In this case if it was left hook the master would be directly behind the tank and the slave would be directly below the master.

Bugger custy wants it pulled apart (deposit paid too).Why can I foresee a large pile of French faeces in the corner of my workshop gathering dust!
thejazzpianoma, Feb 11, 11:25pm
LOL, yes you are right, mechanics strike this sort of thing and then tend to make blanket assumptions that anything from any manufacturer from any European Countryis going to be just as much trouble. which of course is nonsense.

You may well be right about the LHD/RHD thing but sometimes with Renault/Citroen/Peugeot they just don't think. Stupid stuff like not putting a handbrake adjustment access hatch in the centre console on the Xsara, so you have to remove the whole console to adjust the handbrake.

My advice with the Renault is to just roll your sleeves up, rip it apart and then get your parts on the way as quickly as possible. Usually parts are on my doorstep in about a week so it's more of a case of the sooner you start the sooner you finish. But do take a moment to be properly sure you have ordered the right things and everything you need.

I would also google for a transmission removal and/or slave cylinder replacement guide. Chances are fairly good there may be some helpful tricks to it.

Best of luck, give us a yell if there is anything I can do to help.

Oh. and a little psychological advice. as hard as it may be, try and re-frame the image of the steaming pile of digested wine, pastry and snails, into anticipation of an interesting new challenge and learning opportunity. If nothing else its a change from the normal.