could be load sensing valve is siezed or u/s and not allowing fluid through. may be master cylinder faulty.try disconnecting linkage so full pressure go's to rear and now see if they bleed up.
k, January 2012
always start at furthest point from master cyl,make sure rubber hoses is serviceable because they tend to clog up over time,loosen pipes at mastercyl as wel before bleeding to ensure fluid get to main lines
