Bad clutch master cylinder?

[Redtail]

Hey guys, I just wanted to double check my diagnosis...

 

Background: 1973 tii, 5-speed, new clutch slave cylinder, hoses, and reservoir. Been through the bleed cycle multiple times with a pressure bleeder. No fluid loss at all.

 

Issue: Recently the car has started to fully release the clutch while the pedal is still pressed to the floor. For example I'll put the car in reverse and slowly start backing up as normal then the clutch will suddenly fully release and slam the transmission into gear. This happens most noticeably in reverse and first but may be happening with every change.

 

Diagnostic: Internally leaking clutch master cylinder. I replaced the slave (but not the master) about 6 months ago so this might have been the trigger causing the master to fail.

 

Ideas? Does this sound reasonable? Thanks for any advice.

[halboyles]

It is sometimes difficult to pressure bleed the MC like you would a brake line.  Folks often recommend reverse bleeding the clutch hydraulics.

 

Also,  I have had the MC leak but the leaking brake fluid was retained inside the rubber bellows that surrounds the input shaft.  So it doesn't look like you've lost a lot of fluid.  The only way to find out is to pull back the carpet and push the bellows back down the shaft to look for fluid.  

 

 

[Hans]

which way does the bleed nipple point?

[thinksound_mike]

4 hours ago, halboyles said:

It is sometimes difficult to pressure bleed the MC like you would a brake line.  Folks often recommend reverse bleeding the clutch hydraulics.

 

Also,  I have had the MC leak but the leaking brake fluid was retained inside the rubber bellows that surrounds the input shaft.  So it doesn't look like you've lost a lot of fluid.  The only way to find out is to pull back the carpet and push the bellows back down the shaft to look for fluid.  

 

 

I had my clutch catch extremely low after having my transmission replaced. The shop bled the clutch 3-4 times and it still was catching low. I took it home and reverse bled the clutch and the problem went away. Try it and see if it helps. If not, it could be the master cylinder. Brake fluid is way cheaper, so it's worth a shot.

[TobyB]

yes, it's either the master or the slave.

 

If there is no leakage, master is the likely offender.

 

I raced for a weekend like that-  it's a pain...

 

t

[Redtail]

18 hours ago, Hans said:

which way does the bleed nipple point?

the slave cylinder bleed nipple points up so that should be ok.

 

I was able to find a bit of fluid seeping past the seals when I pushed back the rubber boot on the M/C. Seems pretty obvious that's the issue. I have a new one on order, thanks for the advice guys.

[Mike Self]

In a system like the clutch hydraulics where there are only two hydraulic cylinders (master and slave), what you don't replace becomes the weak link, and thus the next failure point.  It appears that's what's happened to you if you can't bleed out the system.  As I'm sure you know bleeding the slave on a 5 speed is much more difficult than on a 4 speed due to the slave's location, so make sure you're good there before replacing the M/C.  But if the M/C is old, then it's probably in the process of failing...

 

 

mike

[Son of Marty]

2 hours ago, Redtail said:

the slave cylinder bleed nipple points up so that should be ok.

 

.

If it's a O/D 5 speed from a 320i the bleed nipple should be at the bottom and the feed hose on the top, I know that it sounds backward but it's not 

[Hans]

Like Son of Marty says! Don't know if that's your problem but it won't help the situation. 

[Redtail]

16 hours ago, Son of Marty said:

If it's a O/D 5 speed from a 320i the bleed nipple should be at the bottom and the feed hose on the top, I know that it sounds backward but it's not 

 

Really? That's the set-up in the car and the bleed nipple is on the top and the hose feed is on the bottom. It was like that when I got it so I just replaced the slave cylinder the same way.

 

Should I switch it around so the nipple points down? It is a PITA to get at with the nipple pointing up.

 

I replaced the M/C and after an extensive bleeding session finally got everything to work. I feel like I could use another round of bleeding but it's ok for now unless I should swap the position of the slave. Thanks guys

[Son of Marty]

Yes, unfortunately to get all the air out you'll need to turn it over, and yes it a major pain, sorry

[Redtail]

Oh crazy! That explains why it's been such a problem to bleed. I was wondering how everyone else got at the bleed nipple with 1/4" of clearance all around!

 

I'll flip it over and bleed again. Hopefully it will make my shifting buttery smooth.

[Hans]

You are not the first person who assumed up is good with the 5 speed slave. Hence my question. Some people use pressure bleeder to bleed backwards ( through nipple).

[Dick R]

After replacing the master and slave clutch cylinders on my 4-spd, I tried two bleeding methods: reverse bleeding and the old clear hose into a fluid catch container on the floor.  Neither worked to my satisfaction.  Then I moved the fluid catch container above the bleed screw - maybe a foot nearly straight above it in the engine compartment.  A few fairly slow clutch pedal pumps and all the bubbles stopped.  With the clear plastic hose going kind of straight up, the bubbles only go up away from the bleeder screw.  That did the trick for my clutch.

 

I have zero experience trying this on a 5-spd.