Clutch master cylinder or Slave cylinder, Which is FAIL ?

Like to see or make a sticky or link about clutch hydraulics diagnosis. Probably could apply to most models. I have a decent understanding of hydraulics/pnuematics.
Assumptions:
This would be specific to the hydraulic end of the clutch (not slippage or chattering)
No hydraulic leaks, if your losing fluid, you have a leak somewhere and are getting air into the system then it is FAIL.
No air in system due to any of the above in the braking/clutch sysytem (B4 brakes and clutch use the same fluid reservior {chime in on other platforms}

Soooo:
I replaced a rear brake line in my B4 and bled the system including the clutch using synthetic brake fluid "meets or exceeds DOT 3&4 standards". Hopefully it meets or exceeds VW standards.
first ~10 mile drive was fine but then the clutch pedal would gradually bleed down to the floor and clutch would engage. I can manually pull the pedal up but the next time I dis-engage the clutch, the pedal will do the same thing.

I bled the clutch again using a piece of tubing fed back to the reservior "brand new fluid" to verify no air in it.
I figure that I need a new clutch master but that it could be the slave as well or both.
My goal is to correctly diagnose which component is FAIL as well as help others from replacing both componets when only one of them is needed. I hope that I haven't "Toofed" "my hydraulic sytem by using synthetic brake fluid "thought it would be an upgrade {like synthetic oil}

Here is a theory. If a slave cylinder has a leak the fluid has to go somewhere. The only where to go is out past the seal and off to the world. A master cylinder, whether brake or clutch can leak out past the seal and on to your feet. Well at least on some cars. Or it can leak internally. So my vote is to get some regular old brake fluid when you change the master cylinder. But I could be wrong and then you just need a slave cylinder. Fluid goes one way in my vehicles. Into a glass jar and off to the recycle center, never back into the system. The risk of contamination is too high. A few dollars of brake fluid is less than any hydraulic part.

roadhard1960 said:

Here is a theory. If a slave cylinder has a leak the fluid has to go somewhere. The only where to go is out past the seal and off to the world. A master cylinder, whether brake or clutch can leak out past the seal and on to your feet. Well at least on some cars. Or it can leak internally. So my vote is to get some regular old brake fluid when you change the master cylinder. But I could be wrong and then you just need a slave cylinder. Fluid goes one way in my vehicles. Into a glass jar and off to the recycle center, never back into the system. The risk of contamination is too high. A few dollars of brake fluid is less than any hydraulic part.

Click to expand...

If the slave cylinder works like a spool valve and not a wheel cylinder or caliper then it is possible that fluid could bleed back through it as it fails.
My next diagnostic step will be to try and see if there is backflow into the reservior with the clutch pedal depressed as it bleeds out. I'll need to get "Sprinta" to help me out with that.

No, it is not a spool valve. It is just like half of an old wheel cylinder, or a single piston disk brake caliper. It is just a bore with an actuating rod and a seal. Fluid is in bore not under pressure. Depress clutch cable causes increase in pressure against actuating rod and seal. If seal works then rod moves. If seal failed or is failing then fluid leaks past seal on into transmission soon to destroy clutch.

roadhard1960 said:

No, it is not a spool valve. It is just like half of an old wheel cylinder, or a single piston disk brake caliper. It is just a bore with an actuating rod and a seal. Fluid is in bore not under pressure. Depress clutch cable causes increase in pressure against actuating rod and seal. If seal works then rod moves. If seal failed or is failing then fluid leaks past seal on into transmission soon to destroy clutch.

Click to expand...

Thanks
That's exactly the info I'm looking for. Since I'm not losing fluid from the reservior it'll probably end up being the master then. I would expect that if the slave was leaking into the bell housing that I would be losing fluid at a pretty good rate since the pedal bleeds down in about 3 seconds after depressing the clutch pedal.

Sprinta was over earlier and helped me confirm the diagnostics on the clutch hydraulics. We topped off the reservior beyond the MAX and observed the level when the clutch pedal is depressed and during the period where it bleeds down.
the change in fluid level clearly drops when the clutch pedal is depressed and clearly goes up when it bleeds down. The level actually goes higher after it bleeds out until the pedal is manually pulled up.
Thanks again to roadhard1960 for the info on the slave cylinder mechanicals. That took some of the guesswork out of it.

Its usually the master, though I tend to do the slave as well as it typically fails not long afterward. The other thing to be aware of is that sometimes when replacing the master, the clutch line decides to never seal again. This is quite common. There is in fact a supercession for some of them. Just keep that in mind in case you run into a leakage issue(ooh....we hate leakage issues...olestra!!). This way you dont wind up like me with no hair as a result of being mocked by the hydraulic system repeatedly.

When the slace goes there is usually fluid leaking from the system and into the bell housing. When the Master goes out you have to pull the pedal up from the floor. Typically I've found that the slave and master will die within a few months of each other if only one is replaced. Remember they both have the same fluid in each other and the seals are similar and the usage is the exact same.

Clutch Master Cylinder?

Well if this isn't timely. My '97 Passat TDI began locking up in gear because of no action with the clutch pedal. Once in gear it would not come out and would die a horrible death in a forced stall. I thought it was finally the clutch giving up (300K+). I replaced just about everything, clutch kit, slave, lever retaining spring, etc and followed the excellent write up I found here, (also did the TB, serp, H20 pump, hoses, belts-a full make over). My son and I fired it up yesterday and it didn't explode/implode. I was sooo happy.
However,...the clutch pedal misery is still with us. I have to reach down to pull the pedal up. Once up, if I shift very gently and quickly to get off the pedal, it returns but is pretty soft. From what you guys have discussed above, I'm pretty convinced I needed a master cylinder all along.
Is this a difficult job?
Will I need a second mortgage to buy special VAG tools-do I need special tools?
Certainly appreciate any help.
John in MD

'81 Rabbit Diesel
'97 Passat TDI
'98 Passat Wagon
'98 Passat Sedan
'98 Cabrio
'99Jetta
More Tools Than Brains

Mcgink said:

Soooo:
I replaced a rear brake line in my B4 and bled the system including the clutch using synthetic brake fluid "meets or exceeds DOT 3&4 standards". Hopefully it meets or exceeds VW standards.

Click to expand...

Maybe not the case, but I thought I should ask it:

This brake fluid you used - I hope it was DOT 5.1 and it says it is compatible with DOT 3 & 4. If it was DOT 5 then you may have created a problem. DOT 5 is silicone based and VW states specifically not to use it. My Bentley says the following:

Do not use silicone-based brake fluid (DOT 5). Even the smallest traces may cause severe corrosion in the brake system

From what I have read, DOT 5.1 is synthetic based and is compatible with DOT 3 & 4 but gives the 500 degree boiling boint.

Looking at brake fluid containers on shelf-DOT 3 is non synthetic,DOT 4 is synthetic(which is what my 98 Jetta needed).....I have never seen the DOT 5 fluid-is that at the dealer?

mrchill said:

Its usually the master, though I tend to do the slave as well as it typically fails not long afterward. The other thing to be aware of is that sometimes when replacing the master, the clutch line decides to never seal again. This is quite common. There is in fact a supercession for some of them. Just keep that in mind in case you run into a leakage issue(ooh....we hate leakage issues...olestra!!). This way you dont wind up like me with no hair as a result of being mocked by the hydraulic system repeatedly.

Click to expand...

So, I replaced both the master and the slave cylinders.
I ended up going this route based on MrChill's advice. If the slave cylinder Fails, then it "Toofs" the clutch. I probably won't be doing any clutches in the driveway . I wouldh've replaced the line as well if I couldh've found one locally. The hydraulic hose from what I could find is a dealer part and $160 . The line I have has some corrosion but I put it back together and so far, no Olestra!! leakage yet .

jconnelly said:

I replaced just about everything, clutch kit, slave, lever retaining spring, etc and followed the I'm pretty convinced I needed a master cylinder all along.
Is this a difficult job?
Will I need a second mortgage to buy special VAG tools-do I need special tools?
Certainly appreciate any help.
John in MD

Click to expand...

Pretty staight forward R&R. Unbolt the coolant reservior and shwing it out of the way to access the master @ the firewall. The worst part of the job for me was getting that plastic clip out of the clutch pedal. You have to "go down under" the dash for that. You'll want to try and reuse the hydraulic hose, just make sure both ends are nice and clean when reinstalling. I used 50 cents worth of clear 1/4" ID tubing from Lowes run up to a jug to bleed it. So, a little over $200 for both cylinders from www.TDIparts.com. Not too bad, but you'll want to make sure that the hydraulic hose doesn't get "Toofed" so that it can be reused. WD-40 is your friend .

starrd said:

Maybe not the case, but I thought I should ask it:

This brake fluid you used - I hope it was DOT 5.1 and it says it is compatible with DOT 3 & 4. If it was DOT 5 then you may have created a problem. DOT 5 is silicone based and VW states specifically not to use it. My Bentley says the following:

Do not use silicone-based brake fluid (DOT 5). Even the smallest traces may cause severe corrosion in the brake system

From what I have read, DOT 5.1 is synthetic based and is compatible with DOT 3 & 4 but gives the 500 degree boiling boint.

Click to expand...

Jug just reads "meets or exceeds Dot 3&4 standards" and is compatable with both. No mention of any silicone in the fine print.

DOT 5 is used in motorcycles.

Thanks for the info here..., the clutch action on my B4 is getting worse.
I knew there was a slave sylinder and changind was needed but did not know of future demise of the master cylinder SHORTLY AFTER the slave stops operating like OEM!

Brake fluid; corrosive, it IS...

Any hints on getting the master cylinder "ram" removed from the clutch pedal??

The rest has been very straight forward..

Press in on both sides of the white tab at the clutch pedal junction and it'll pop right out.

I found a this in another thread..

"The inside end of the rod is held in with a clip that you squeeze the sides of to release from the pedal. It'll then shift forward and come right out. Only takes a minute or two to replace. I use a thin screwdriver to release one side then the other. Once you push the tab, it'll move forward slightly and stay unclipped.

To put the new one in place, move it into position and make sure the orientation inside is correct, then bolt it down under the hood. Then press (spelling correction) slightly on the pedal and it'll seat the clip. When you hear it click, you're done.

(THANKS ABACUS!)

twice.. LOL

ajrn said:

I found a this in another thread..

"The inside end of the rod is held in with a clip that you squeeze the sides of to release from the pedal. It'll then shift forward and come right out. Only takes a minute or two to replace. I use a thin screwdriver to release one side then the other. Once you push the tab, it'll move forward slightly and stay unclipped.

To put the new one in place, move it into position and make sure the orientation inside is correct, then bolt it down under the hood. Then press (spelling correction) slightly on the pedal and it'll seat the clip. When you hear it click, you're done.

(THANKS ABACUS!)

Click to expand...

This sounds so easy. Mine must be stuck, because I can't get that white clip out. Any word on whether not this clip can be purchased separately since I will likely mangle mine removing it from the clutch pedal assembly?

bblume said:

This sounds so easy. Mine must be stuck, because I can't get that white clip out. Any word on whether not this clip can be purchased separately since I will likely mangle mine removing it from the clutch pedal assembly?

Click to expand...

It's a tough one to get out. The problem really is that access to the left side of the clip is restricted so getting in there to release it is a pita.

The clip part number is 1H0721357 but I'm not sure if it's still available.

Steve

Steve Addy said:

It's a tough one to get out. The problem really is that access to the left side of the clip is restricted so getting in there to release it is a pita.

The clip part number is 1H0721357 but I'm not sure if it's still available.

Steve

Click to expand...

I just found the clip at GAP for $4ish
Perhaps I'll try again tomorrow
I had to stop today before I broke something!
Gotta love wrenching...

I can't remember exactly what I did to remove mine, but I did manage to get it out intact in a couple minutes once I got my stuff rigged up.

Pretty sure I used a light-duty bar clamp, with the "teeth" of the clamp pushing on something that I found lying on my workbench that was about the right size - maybe like a wedge of scrap hardwood flooring or something. Once I was able to hold it all in place (why, God, why.... weren't we born with three hands??!!), a few squeezes on the bar clamp, a little wiggling of things to make sure it stayed aligned, and out it popped.

This is all encouraging while at the same time making me feel worse since I didn't get it out yesterday. Of course I didn't release the spring above that keeps tension on the clutch. Wondering if that would help (assuming I can get that clip off with breaking it?!!)?
Probably try again next week as this cluster is in a donor passat for a 1Z Mk2 project...