Brake bleeding problem

[dave123]

1975 Cj drum brakes, not power assisted

Yesterday, I took the Jeep out. The brakes have been working great, but this time, the pedal went almost to the floor. It was a semi emergency, so I drove the Jeep a little ways like this with no issues rather than the pedal dropping so far down. The jeep did stop, and the pedal did not quite get to the floor. I checked the fluid when I got to my destination, and it was very very low. It did not appear to have exposed the holes to the brake lines, but since I had been driving, you never know if is sloshed around or whatever.

I added fluid and pumped it a couple times, and the pedal appears to have returned to normal. I drove the Jeep again, and the brakes feel good as new.

Now I always thought that you will have good pedal and brakes up until you suck air, then it will drop and get spongy, and even if you add fluid, it won't be normal unless you bleed the brakes.

So, why would my pedal drop if I still had pressure? And why would it return just by adding fluid?

I'm confused, should I do a "just in case" bleed"? Don't really want to unless absolutley necessary, the bleed screws are a pain, and I have to drag the wife outside in the cold.

Any informed advice appreciated!

dave

 

[Petescj]

Had this happen once the way you describe. If you havent been under the hood in awhile you may not have noticed the brake fluid leaking through the top of the cap. Theres a machined surface that mates with the gasket in the cap. Mine had some rust on it wich I guess left enough room for the fluid to pass between when under pressure.

I cleaned the rust off the machined surface but couldnt find another gasket. I cleaned that up and kept an eye on it for awhile after that and it didnt leak anymore. Was the master cylinder wet at all?

 

[jeffgtoman]

Your pedal went down too far because the fluid got too low to supply the power piston. You would have pushed some air into the lines at this point. Your brake pedal came back to normal after you added fluid because the air in the lines was close to the master cylinder. Once you had fluid back in the master cylinder and made a couple of pumps, the air came out of the lines by floating up and going to the top of the master cylinder. There is a foot or more of decline in the lines below the master cylinder. As long as the air doesn't get pushed beyond that point or beyond the proportional valve, it will float up. That is why you can change the master cylinder and not have to bleed the lines. You bench bleed the master cylinder to get the air out of the piston chamber. Then whatever air gets in the lines close to the master cylinder will be pushed into the master cylinder after the first couple of pedal strokes. I hope this helps and doesn't confuse you.

I have to ask though, why did you run out of brake fluid? You should either have wet rear brake shoes, a wet spot along the brake lines or like Petesjc asked. Is the master cylinder wet from a lid gasket leaking.

 

[dave123]

Thank you guys. I know it must be leaking from somewhere, I will investigate.

I guess the answer is I can avoid the bleed, at least until I figure where the problem is. Good.

 

[dave123]

Found a leaking wheel cylinder. Went ahead and did a complete brake job and replaced the wheel cylinder. Looking good now.

thanx