Jump to content
  • 0

Air in Clutch Hydraulics


georg

Question

After accidentally introducing air into the hydraulic system and bleeding it out by pumping the pedal (pump and hold the pedal with a piece of lumber, go under truck and open and close bleeder, release pedal), I will have decent pedal on my drive home. The next day I won't have full pedal and have to double pump the pedal to engage reverse or first. Bleeding again restores the pedal but the day after I end up with a soft pedal and have to bleed again. I have to bleed for about five days in a row before I end up with a permanent good pedal.

 

Exact same thing happened three years ago. I use my truck as a guinea pig to try out reverse bleeding, vacuum bleeding etc which is how I end up introducing air into the system, in case you are wondering why I end up with air in the system.

 

My question is, why do I temporarily have a good pedal after bleeding? After an hours drive and

sitting overnight, the pedal goes mushy again. Shouldn't the air left in the system immediately after bleeding not affect it right away, causing a mushy pedal?

Link to comment
Share on other sites

Recommended Posts

Last time I did a clutch slave bleed it was one of those that was mounted on the outside of bell housing. I unbolted it, and put a tie wrap around the cylinder to hold the piston in place. Used my thumb over the end of the already loosened bleeder. Had assistant just press slowly on the pedal, and I let the fluid and air come out as the pedal went down. When I asked the guy giving me a hand to help, I also explained what I wanted him to do. Told him to press the pedal down slowly, and stop at halfway point. Once he heard me say ok, he was to slowly let the pedal back up, and we repeated that until no more air. Doing it this way also allows you to rotate the slave to get the best angle for the air to exit.

 

This does not work very well on the slaves that are sitting on the throwout bearing support sleeve on the transmission though.

Link to comment
Share on other sites

Re leaving the m/c applied overnight, I had someone else recommend the same procedure for my clutch. It is hard to see how it helps since there is no port that is open only with m/c applied. In fact the compensating port (passage between reservoir and cylinder) is only open when the m/c is released on this truck. There is a valve built in into this port according to the diagram of the m/c in the FSM. My concern with trying this procedure is that if the slave somehow gets damaged by this procedure (after all it is not designed for continous application) I will have to pull the tranny.

 

I tried applying manifold vacuum to the reservoir and it did not help. Also tried a Mity-Vac on the reservoir. I will use the Mity-Vac on the bleeder screw on the slave like you recommend and see if it helps.

Link to comment
Share on other sites

On some vehicles, the Mityvac just won't get all the air out - need a combo of both.

 

That '02 1500 I worked on this week, I was able to completely bleed solo using the Mityvac. Some work, some don't - it's a crapshoot.

Link to comment
Share on other sites

My previous truck was a Ford F250 and it had an external slave. I would unbolt it and holding it so that the line connection was uppermost, would slowly stroke the push rod and all the air would get driven into the master cylinder reservoir. I did not even have to open the bleeder screw.

 

I am beginning to think pedal pumping is not a good way to bleed a clutch since once you open the bleeder and drain out some fluid and then close it, a full vacuum develops in the system while the pedal is being released. Perhaps a bit of air leaks in past the seals into the master cylinder since the seals are designed to hold in pressure, not vacuum. With brakes there are a lot more lines and flex hoses, not to mention calipers and wheel cylinders, just a lot more volume to the system and that prevents vacuum from developing.

Link to comment
Share on other sites

My vacuum gauge isn't a Mityvac but it is US made (no brand name) and has lasted for many years without any issue.

 

Today was the first day in a few days when the pedal did not go mushy over night. Yesterday was also the first time I did not pump the pedal and bleed but used the vacuum pump.

 

I suspect air was being drawn in past the seals into the master cylinder when the pedal was retracted and a full vacuum was created inside the m/c. From now on, for clutches, I will only do either a pressure bleed or a reverse bleed. No more using the pedal to bleed.

 

I work on HD trucks and their clutches gravity bleed easily, expelling any air in the process.

Link to comment
Share on other sites

Another trick I use on bikes (and I'd assume it works on cars/trucks too but i can't verify that because I can't be 2 places at once) is after the vehicle has sat any length of time after bleeding, when you first get in, just LIGHTLY touch the brake pedal, moving it maybe 1/8" or a couple mm. On bikes I always see a stream of bubbles come out of the master when doing this. After doing this several times, you can get all the air out without even cracking the bleeder again. On bikes I flick or tap the lines to force the bubbles upward that may be trapped.Takes some patience, but it works.

 

Funny we're on this subject today. Yesterday I completed a 2 day horror show on a '02 Silverado 1500 4x4. Replaced 24' of brake line up front to both front wheels and from the master to the ABS unit on the frame rail. Then in attempting to purge trapped air in the rear lines, one bleeder rounded off, and the other snapped. Rather than screw with it, I hopped on the RD350 and smoked my way to and fro the auto parts store. Total rot box. What a nightmare. Glad that's done!

Link to comment
Share on other sites

Ok, I will try lightly working the brake pedal tomorrow, before vacuum bleeding the slave.

 

I don't want rusted out brake lines on my truck, so I have been meticulously keeping the lines sprayed with rustproofing oil (Krown). I get the truck sprayed at a local Krown shop every two years but they don't pay much attention to the brake lines and fuel lines. So, every spring and fall, I get under the truck and spray Krown on the brake and fuel lines, especially the tangle of brake lines near the ABS pump.

 

I wonder what the dealership technicians do about bleeding the clutch since they cannot do it repeatedly over a few days. Google searching brought up a number of guys complaining about having to bleed the GM truck clutches multiple times.

 

I don't have any issue bleeding the brakes on my truck, just once does it. I don't think that my bleeding method is flawed.

Link to comment
Share on other sites

My previous truck was a Ford F250 and it had an external slave. I would unbolt it and holding it so that the line connection was uppermost, would slowly stroke the push rod and all the air would get driven into the master cylinder reservoir. I did not even have to open the bleeder screw.

 

I am beginning to think pedal pumping is not a good way to bleed a clutch since once you open the bleeder and drain out some fluid and then close it, a full vacuum develops in the system while the pedal is being released. Perhaps a bit of air leaks in past the seals into the master cylinder since the seals are designed to hold in pressure, not vacuum. With brakes there are a lot more lines and flex hoses, not to mention calipers and wheel cylinders, just a lot more volume to the system and that prevents vacuum from developing.

 

Your last paragraph is why I would only go halfway down on pedal, and assistant would slowly let pedal up. I have also used the tried and true method of putting some teflon tape around bleeder screw threads, put a piece of clear tubing over the end of the bleeder, and hang the other end into a bottle half full of brake fluid. With bleeder open(actually it was not really able to be seated due to teflon tape) you just slowly press the pedal all the way down, and slowly release it. It would push air and fluid into jar, then when you released the pedal it would pull fluid back in. Takes a bit longer this way because the air has to fully exit the tube. If you keep the tube as short as possible it works better.

 

I have a kit like this one, difference being mine is about 25 years old and has a rebuildable metal vacuum pump. Brand name is Mity-vac.

http://www.amazon.ca/Mityvac-MV8000-Automotive-Test-Bleeding/dp/B00265M9SS

Link to comment
Share on other sites

Re leaving the m/c applied overnight, I had someone else recommend the same procedure for my clutch. It is hard to see how it helps since there is no port that is open only with m/c applied. In fact the compensating port (passage between reservoir and cylinder) is only open when the m/c is released on this truck. There is a valve built in into this port according to the diagram of the m/c in the FSM. My concern with trying this procedure is that if the slave somehow gets damaged by this procedure (after all it is not designed for continous application) I will have to pull the tranny.

 

I tried applying manifold vacuum to the reservoir and it did not help. Also tried a Mity-Vac on the reservoir. I will use the Mity-Vac on the bleeder screw on the slave like you recommend and see if it helps.

 

The idea is that when you apply pressure to the pedal(or lever when on a motorcycle) it somehow encourages the bubbles to travel up to the highest point. Once the pedal or lever is released, the compenating port opens, and the air bubbles back into the resevoir. The clutch master cylinder is the same concept as the brake master.

 

The reason for the compensating port is to allow fluid to go back into resevoir with the pedal released if the fluid has been heated enough to expand. If you ever manage to misadjust the pushrod between the pedal and master cylinder and make it too long, you will be lucky to make it a mile or two.

 

Had this exact problem on my 85 VF1000R Honda after the restoration. The NOS brake and clutch lever both had incorrect tabs on the lever that applied pressure to the piston in the masters. Made it to the second set of lights, brakes were getting hot and were applying enough pressure, making them get hotter, etc, and the clutch started to slip. Luckily the clutch did not get hot enough to damage anything.

 

Lesson learned, just because someone says they are NOS does not mean shit.

Link to comment
Share on other sites

I vacuum bled my truck today for just five seconds and now I have full pedal. What is not clear is if it will get mushy again.

 

While looking around under the truck I noticed that the clutch line does not go uphill all the way from the slave to the master cyl. The line actually dips to clear the driver side cat. This dip probably makes it more work to bleed the line than it would be if the line ran uphill all the way to the m/c. At least the dip is not as bad as its opposite - a section of line higher than its two sides. I believe Ford Rangers have this air trap and makes it really difficult to get all the air out.

Link to comment
Share on other sites

Techs always have another body around to help push the pedal while they do their thing. We don't have that luxury ... unless the wife is home early from work in my case. With a Mityvac plus another body, the air gets out pretty quick.

Link to comment
Share on other sites

I get to do a slave cylinder in a '94 S10 I just got for free, whenever I get a minute.

 

For some reason those were always more difficult to bleed than brakes, probably because they always mounted the reservoir lower on the firewall. Not sure how they do things these days. Last one I did was in a '83 Toyota pickup, many many years ago.

Link to comment
Share on other sites

My vacuum gauge isn't a Mityvac but it is US made (no brand name) and has lasted for many years without any issue.

 

Today was the first day in a few days when the pedal did not go mushy over night. Yesterday was also the first time I did not pump the pedal and bleed but used the vacuum pump.

 

I suspect air was being drawn in past the seals into the master cylinder when the pedal was retracted and a full vacuum was created inside the m/c. From now on, for clutches, I will only do either a pressure bleed or a reverse bleed. No more using the pedal to bleed.

 

I work on HD trucks and their clutches gravity bleed easily, expelling any air in the process.

 

I may be wrong, my memory sucks(other than remembering I have a crappy memory), but I seem to recall there being two opposing seals on the rear of the first piston(the one under the rear reservoir). It was done to prevent what you are thinking is happening. This was back in the days when you would rebuild master cylinders, wheel cylinders, and calipers.

Link to comment
Share on other sites

I have the same one, only mine is metal as well.

 

Mine is a bit newer, at 16 years old. Haven't rebuilt it once yet! I have a rebuild kit sitting in my tool box unused. I have had it apart several times though, as it would stick after a period of non-use, and then bleed vacuum down when you tried to hold it. I've managed to mickey-mouse the thing along for the past 11 years. I can get down to 27" of vacuum now - couldn't do that when it was new! :)

post-65085-0-69043600-1439820359_thumb.jpg

post-65085-0-96052800-1439820379_thumb.jpg

post-65085-0-43201900-1439820396_thumb.jpg

post-65085-0-69043600-1439820359_thumb.jpg

post-65085-0-96052800-1439820379_thumb.jpg

post-65085-0-43201900-1439820396_thumb.jpg

post-65085-0-69043600-1439820359_thumb.jpg

post-65085-0-96052800-1439820379_thumb.jpg

post-65085-0-43201900-1439820396_thumb.jpg

post-65085-0-69043600-1439820359_thumb.jpg

post-65085-0-96052800-1439820379_thumb.jpg

post-65085-0-43201900-1439820396_thumb.jpg

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.



×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.