Clutch bleed failure

[intoflatlines]

I got air into the clutch system during my clutch bleed. The reservoir level dropped much quicker than I anticipated and now the clutch pedal easily goes to the floor without much effort at all. Can this be solved with (careful) re-bleeding? Or am I screwed and have to pull the master/slave cylinders?

[inTgr8r]

The best way to do this is from the bottom up.
Pump the fluid in at the bleed forcing it up into the reservoir.
The air will rise to the top ahead of the fluid
You can use a large pump oil can to pump it in.

[DanFromTucson]

You can carefully re-bleed it. Are you using a pressure bleeder, or just the manual foot-to-pedal approach?

If you think air is trapped in the slave cylinder and not bleeding out, you can remove the slave cylinder pretty easily and tilt it so that all the bubbles float towards the bleed valve.

Also, if your brake fluid reservoir went dry, you should make sure that your actual brake lines didn't suck in any air.

[intoflatlines]

Used a pressure bleeder last night. Trying a vacuum bleeder today to easier monitor reservoir level. I don't know how to remove the slave cylinder though. Brake fluid reservoir did not go dry, it just went down enough to expose the clutch tube to air.

[Shipkiller]

I would not worry too much about the brakes since the clutch supply line is 1" above the bottom of the reservoir, unless you KNOW you drained the reservoir.

I have been where you are.....

A pressure bleeder will not completely get all the air out of the system.

Tried a pressure bleeder, did not work. I tried pushing three quarts of fluid through it... no go.

Used the gravity method, did not work.

Tried the oil can method, did not work.

What DID work was the two person method...

[Clearancediver]

Out of curiosity what do you mean by two person method. Old school method where buddy pushs on clutch pedal while 2nd man opens/closes bleeder? That's the same as using a vacuum bleeder .... Air pressure system that supplies a vacuum to the bleeder and you simply open the bleeder and watch the fluid drain. A reservoir with fluid that's inserted into the cars brake/clutch reservoir supplies fluid so the reservoir doesn't go dry. My mechanic son introduced me to it doing a brake line bleed on my old car. I did the work...he supervised....to his amusement.

The only difference that I can see is that applying pressure somehow "moves" air that's trapped in high spots when the bleeder is opened. But a vacuum should work just as effectively.

Hmmm... Thinking here....(diving experience )...Pressure will cause air bubble to shrink... Void fills with more fluid... Repetitive pressurization/release of pressure might "ease air out of a void space that's trapping air..... Vacuum at bleeder doesn't provide the same effect.....

Plausible explanation on why old school 2 person method worked in this circumstance...

[intoflatlines]

I used a vacuum bleeder and it seems to have worked. I can't tell a big difference with the CDV delete though!

[Shipkiller]

When I had my issue, I researched the hell out of it.

It seems that all BMW slave cylinders are constructed similar and have the same issue when bleeding. There seems to be small pockets internal to the cylinder that trap a bubble or two.

In theory, you would think a pressure bleeder would push the bubbles out, but that did not happen in practice.. I tried a pressure bleeder with the front of the car raised above the rear, and with the rear raised above the front and with the car level. No difference.

The vacuum method works in the reverse and uses less force. I've used this method before. It did not work in this instance.

More research followed and I found the 'oil can' method.. no luck using that method either.

Using the old fashioned two person method, being very precise and controlled with each step worked like a charm. It took about 20 peddle pumps until I could not detect any of the TINY bubbles being purged.

[intoflatlines]

If I had someone to help I would probably have tried the standard 2 person procedure. I will likely do that next time. But yeah... I didn't notice a big difference from the CDV delete. Not sure what it was supposed to do

[elerner61]

The CDV delete is a subtle change. It makes the "why the hell did the clutch/car do that when I thought I was being very smooth" moments go away.

[Clearancediver]

The two man method is going to keep the brake or clutch line pressurized considerably more than a system that is pushing fluid into the system. Boyles Law will apply to the reduction in size of air in the system... Twice atmospheric(14.696 psi ).. 1/2 original size... Three times atmospheric ...1/3 original size. Pressing on the brake or clutch pedal and the second man starting a slight bleed will keep the internal pressure at least 2-3 times atmospheric. The air in the void or bubble is kept smaller and will move easier. There are no doubt other fluid dynamics involved internally wrt pressure drops... http://hyperphysics.phy-astr.gsu.edu/hbase/pman.html

[RADRACR]

I've attempted speed bleeders and pressure bleeding and I've failed miserably on all accounts. Two-man (or in my case, man+wife) is the only method that works every time. But man, getting at that clutch nipple is a BITCH.

[DanFromTucson]

Originally Posted by Shipkiller

There seems to be small pockets internal to the cylinder that trap a bubble or two.

This is why I had to unbolt it and tilt it so that the bubbles went out the bleed valve. It's just held on by 2 bolts. They are a bit hard to reach, but if you have a variety of wrenches it shouldn't be too hard.

[Finnegan]

Originally Posted by DanFromTucson

This is why I had to unbolt it and tilt it so that the bubbles went out the bleed valve. It's just held on by 2 bolts. They are a bit hard to reach, but if you have a variety of wrenches it shouldn't be too hard.

Creative solution, good thinking!