evil_goat

On my 1992 Cherokee without ABS, my brake fluid was black, so I bled it out. However, on all 4 wheels, whenever I had my helper press the pedal, I saw some small bubbles in the hose, particularly with the clean fluid. Sometimes there was a bubble that was 1mm in diameter. I then gravity bled, and a small amount of bubbles for a few seconds, then none for the next 30s. I used about a liter of fluid in total. What could cause bubbles in the fluid? Were the bubbles already there? The hose was on the bleeder valve pretty good, and I closed the bleeder before my helper released the pedal. When I started the car, it seemed easier to push the brake pedal to the floor in park, but when driving, the pedal felt similar to before. The car seemed to stop better when taking it for a 20 min drive, and stopping several times. If there are still air bubbles, would the brakes get worse?

Jim Malcolm

After replacing the entire brake system on mine, I had hell to pay to get all the air out. I bled that sucker every way imaginable except pressurizing the reservoir. I eventually got all the air out but I put at least 2 quarts through it. In retrospect I probably did not bleed the "proportioning valve" correctly.

But anyway, that's a long way to ask, is it possible that the MC or steel lines were replaced at some point in the past by a PO and just not completely bled? I take your description to indicate that nothing was done as part of this go-around. But with the amount of bleeding I did, it's very easy to image that someone else would not have done so...

With the pressure that the brake system operates as, it's extremely unlikely that it would pull in air, but not leak out fluid. If it's leaking fluid, it's not building pressure. If you're only doing this because the fluid was old, I'm leaning towards a non-issue. Crazier things have happened though; I'd give it real good look for leaks...

evil_goat

It's possible the brake lines were changed, but it would have been 20+ years ago, and PO was a mechanic. I'm only replacing the fluid because it's old and doesn't stop as well as my other newer car. Before I bled, the fluid level was about the same as 2 years ago when I topped it off indicating no leaks. The brake pedal feels spongier compared to other cars, and maybe a little more spongy after bleeding, however, it does seem to stop better now. Can air get in through the screw threads of the valve? Should I try bleeding though another liter?

lawsoncl

If I'm vacuum or gravity bleeding, it really helps to put a smear of anti-seize on the threads or at least put a glob of grease around the base of the bleeder to prevent sucking air in around the threads.

Jim Malcolm

Brake systems operate at several hundred PSI while the pressure of the air trying to get in in is only 14.7 psi. While crazier things have happened, it's exceedingly unlikely that it will pull air in without oil leaking out...

Now if you're talking about around the threads while trying to vacuum bleed, absolutely... I personally just loosen the bleeder enough to let fluid flow thru the threads and then tighten it down, using the fluid to seal the threads. Lawsoncl's suggestion to use grease or anti-seize works just as well.

Dave51

Assuming you still have rear drum brakes, are they properly adjusted, or even a little (temporarily) on the tight side? If they're way out of whack, your bleeding and stopping will both be less than efficient.

lawsoncl

True. My suggestion was when doing gravity or suction bleeding.

Jake Rainwater

The bubbles are air in the brake system. When you bled the brakes did you have the person pressing the brakes hold the pedal down when the bleeder screw was tightened? If not that causes air to enter through the bleeder screw and you have to bleed it again.

dmoe69

I removed my comment. For the second time this week I made a comment that had nothing to do with the Thread starter's question.

5-Speed

This is the way. I will not bleed brakes any other way. The big plus is that you dont even need a helper.