Okay, so I have been in the process of rebuilding my front end prior to a full-on slip yoke eliminator/double cardan driveshaft install with a replacement rear pinion seal tossed in for good measure. This will quickly be followed by a replacement lift kit. (The one the PO slapped on here sucks hard and all the shocks are blown out, which makes it even worse.)
Yesterday I finished the main bit of the front end on the passenger side: U joint, ball joints, unit bearing, rotor, pads, caliper hardware. Very nice.
Today I compressed the piston of the driver side caliper and brake fluid started coming out from under the plastic reservoir cap.
I do not do brakes normally, just pads and rotors like your average DIY car person, so this is a new one for me. So I am fairly certain that I should have done something simple like remove the cap or whatever, failed to do so and have now blown my master cylinder or booster or a steel line or connection; something potentially bad. This is the way of things for me, of course. ;-)
What I saw - for certain - was fluid escaping from under the yellow cap. It ran everywhere. I moped it up and sprayed a bit of brake cleaner around the area to dry it out and look for the source in case it was not from the cap.
I got all the brake fluid up, avoided spraying the electrical connector and wires under the master cylinder, and cleaned up the area pretty well. I did not note any more leakage.
I removed the cap after about a half hour and used paper towels to soak up a bunch of fluid from the overly full reservoir. (Not too worried about paper fibers. I am about to do a power bleed of the system after all this work is done. I first have to install new lines and calipers up front, though, so the TJ will be getting all new brake fluid soon and stray paper towel fibers do not worry me.)
I have not yet tried to depress the brake pedal to search for the leak. I was mainly concerned with getting up all the dripping fluid from painted areas and wiring. The piston in the driver side caliper is still compressed.
After the piston was fully compressed back into the caliper body and this happened, after all the cleanup, I noted that the level of the reservoir was overflowing. It was over the FULL line and coming out the top. I had been so occupied with cleaning up the spillage that I had not noticed the level of the brake fluid.
My guess is that these worn pads and rotors allowed the pistons to be pretty extended all the time, making the fluid level seem to be low. The PO filled it to the mark. I came along and installed new, thick rotors and pads, and now it is way too full. So when I compressed the piston the excess fluid blew past the plastic cap's seal.
One of the metal lines looked wet where it entered the master cylinder, but that was probably just runoff from the cap directly above the line. Fluid got all over the place in that immediate area. It did not spray, but it ran out the edge of the cap in all directions.
So, have I fried my master cylinder or booster, or possibly blown a connection or hard line?
After I get the front end back together tomorrow I will try the brakes gently to see if the cap continues to leak. The piston will be able to extend a bit, so the level should drop back down some.
It did not do this when I did the passenger side, which seems odd to me...
Perhaps the two combined to blow the rubber seal on the cap? Could it be that all I need is a new cap or is this a sign of something more serious? I have little understanding of the brake system but have compressed a lot of pistons to install pads over the years and never had this happen, so I am a bit freaked out.