I keep having trouble with my brakes. Randomly they'll just stop working and the pedal will go to the floorboard and I have to pump them to build up pressure. Sometimes I get a brake warning light and sometimes I don't.

The front discs and rear drums seem to be in good shape. I replaced discs, drums, all the pads, shoes and master cylinder within the last couple of years and nothing is excessively worn. The only thing that seems to fix it is bleeding the system back to front. They work great after that for quite a while. When they go out again it's sudden and doesn't happen gradually. My XJ has 160k miles.

Any ideas?

 

Are you losing any break fluid? If they only work right after you bleed them it sounds like you are pulling in air from some place I would start by checking all the break lines, The bleeder screws that they are tight. Also check the seal for the master cyl. cap. That is what I would start with. Then you can move on with the master cyl itself.

 

I know this sounds like way too obvious of a thing - but are you bleeding them from the bottom up? Your system sounds like it has air trapped in there somewhere, and if there's no noticeable leak, then it seems like there's an air pocket in your line somewhere.

 

X2. It does sound like you have air pockets in your lines. It has happened to mine before, and it turned out my mechanic forgot to bleed the lines after doing a brake job. Sounds like all it is, is air. :-)

 

Was the master cylinder bench bled?

 

I agree it sounds and "feels" like there's air in the lines but the reservoir is full and no bubbles come through when I bleed it. Haven't found any external leaks anywhere. I thought the master cylinder had an internal leak which is why I replaced it. I bench bled it.

If its leaking down then shouldn't they seem to go out slowly instead of overnight?

I think I'll replace the rear cylinders and the soft lines with braided steel.