Hi,
99 xj with abs, problem is that the rear wheels lock up before the front wheels causing the abs to always fire when reaching a low speed (no brake light or abs light). I purchased a proportioning valve and want to replace it.
When replacing the proportioning valve do i have to bleed the abs system (with a scan tool) or is a base bleed good enough?
Also what else can cause this problem (my front tires are 205 75 15 rear are 215 75 15) can that cause a problem as well?

 

Orrrr you could remove the fuses for abs to eliminate it all together at least that's how mine is and it still works and stops beautifully! Although that probably isn't the route you want to go, in which case I don't have any further knowledge on abs. Sorry I couldnt be of more help

 

Yes, removing the ABS fuse will still allow you to have normal braking untill you can fully diagnose and repair your problem.

 

It would be the other way around would be the physics if it. The smaller would lock up first. (just picture a bigger difference, say 12in on one and 36in on another). The one with the shorter radius would have better leverage to lock.
(that's with all else being equal, and I think the fronts get more)

 

Yes, the drums should lock up first. Due to the wheel cylinder being smaller and drums "self energize" under braking.