Weird clutch, foot peddle play issue

My 85 manual trans jeep sat up for a good 9 months last year, and the slave cyl was not happy about it. I cleaned it up and bleed the system, put in new fluid about 3-4 months ago, been driving it about 3 times a week to twice a month. Today was first in a week, and the last 2 weeks the foot peddle was nearly to the floor again, but like before 6-12 pumps of the peddle first use of the day, and it was good to go all day. Today, if I was in 4th or 5th gear for 5-10 minutes on the freeway, and had to down shift, I had to pump 5-6 times before I had enough peddle to down shift. Then it was OK for 4-5 minutes again. I pull in the zone for some stuff, and I pull the cap on the clutch MC, and yep it is still full (and already brown), and I reinstall the cap. Go inside and do the shoping, return, and take off, and guess what??????????

The clutch is now acting like a brand MC/SC rig, only about 1/2" of foot peddle play before I get solid clutch action.


????????????????????????

I am sort of scratching my head on this one. Guessing an air bubble was trapped and some how escaped???? But not sure how or where. I did not see any bubble when I pulled the cap.

The old parts are 5 year old rebuilts. I have new ones ready go in, and I can see tiny signs of early seal leaks on the old ones, from where the shaft rusted setting up for nearly a year llast year, which is now where it wants to leak ever so slightly over the months. But the reservoir was still full from about 4 months ago when I flushed and refilled it.

Thoughts????

Clogged vent in cap?

I'm just guessing, but the volume of fluid pumped in one stroke of the pedal is so small that it might not take much to hold it back.

My best guess would be a defective master cylinder. With brakes, sometimes, it makes a difference how quickly you push the pedal, because a fast push will expand worn cups a bit and catch better. You might also experiiment with holding the clutch down and see if it gradually starts to drag. If so, then the MC is leaking down.

Does the cap have a vent? One way?, Two way? I thought the cap was to keep air and thus water, out?

I replaced both last night. Looked like the slave was the only one leaking.

Two things I've noticed about slave cylinders is, one, the Dot 3 attacks the cylinder walls. And whatever leaks out also lets some air in. Air is a whole lot thinner than DOT 3 and when the fluid leaks down it isn't totally a one way process. A little air gets back into the system, a tiny amount on multiple occasions eventually adds up, where it eventually ends up is anybodies guess.
I drained the slave in my YJ into a clear glass jar, let it settle, I was looking for "O" ring particles. There was actually a super thin layer of aluminum in there. DOT 3 seems to be pretty darned corrosive.

8Mud said:

Two things I've noticed about slave cylinders is, one, the Dot 3 attacks the cylinder walls. And whatever leaks out also lets some air in. Air is a whole lot thinner than DOT 3 and when the fluid leaks down it isn't totally a one way process. A little air gets back into the system, a tiny amount on multiple occasions eventually adds up, where it eventually ends up is anybodies guess.
I drained the slave in my YJ into a clear glass jar, let it settle, I was looking for "O" ring particles. There was actually a super thin layer of aluminum in there. DOT 3 seems to be pretty darned corrosive.

Click to expand...

I refuse to use aluminum cylinders, MC or SC, an any of my vehicles for that very reason!!!! I had two less than 1 year old, MC's wipe out good working Vacuum boosters, when they leaked out the rear.

All the DOT fluids are extremely hygroscopic, they suck water right out of the air (at least here with 200% humid Houston, LOL, they do). And that makes it even more corrosive!!!

Ecomike said:

Does the cap have a vent? One way?, Two way? I thought the cap was to keep air and thus water, out?

I replaced both last night. Looked like the slave was the only one leaking.

Click to expand...

Usually the cap will have a diaphragm in it, with a vent on the atmosphere side. There has to be some way to prevent suction when the fluid goes down.

Matthew Currie said:

Usually the cap will have a diaphragm in it, with a vent on the atmosphere side. There has to be some way to prevent suction when the fluid goes down.

Click to expand...

I would take it that it is a closed sytem, which rarely vents, but maybe has a diaphragm in the cap that follows the fluid up and down, with the top of the diaphragm open to the atmosphere, as it is a no-no to let the hydraulic fluid see air, because it will suck water out of the air and ruin the hydraulic fluid. Water lowers the boiling point and makes it corrosive.

Ecomike said:

I would take it that it is a closed sytem, which rarely vents, but maybe has a diaphragm in the cap that follows the fluid up and down, with the top of the diaphragm open to the atmosphere, as it is a no-no to let the hydraulic fluid see air, because it will suck water out of the air and ruin the hydraulic fluid. Water lowers the boiling point and makes it corrosive.

Click to expand...

Quite. I was surmising that if the outside vent was plugged, perhaps the diaphragm would cause a little vacuum when the fluid goes down, just enough to cause problems in a clutch system, which tends to have a very small displacement and is very sensitive. It's a long shot, but if there were no other signs of leakage or failure it seems worth considering if it recovers after you take the cap off.