Hydraulic brake system diagram- where to find?

Does anyone know of a ULR or other location where I can see a schematic diagram of a non-abs XJ hydraulic brake diagram. I'd like to see what pistons, valves, and so on are inside the master cylinder and the proportioning valve assembly.

I have a 1992 XJ auto-trans I-6 which had the stock ABS. This unit came on unexpectedly and held slight pressure on the system. Burned up my front rotors. Swapped in a D-44 in the back which had no sensors so out went the ABS. I yanked the unit, bought a junkyard proportioning valve and plumbed like the junkyard vehicle. Works great. Except sometimes, and only occasionally, if I place really gentle pressure on the brake pedal the pedal will go almost to the floor and the BRAKE light will come on. I still stop and I think the front system is working but somehow the back system isn't getting pressure. There is no hydraulic fluid consumption or leak so there must be a valve or seal in there somewhere that doesn't seal sometimes. System works great under quick strong pedal pressure. I've blead the system of course.

So if I can find a schematic diagram maybe I can understand which seal is flakey.

Thanks!

Mike

are you sure there isn't air or something else in the system?

Pascal's law or the Principle of transmission of fluid-pressure states that "pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid."

Ok, somehow your system is occasionally not following Pascal's Law. If there was air in the system it would be a constant and repeatable condition all of the time. If we accept your statements, then the ONLY place it could happen "occasionally" is in the MC if pressure/fluid is able to bypass the piston and return to the MC. Based on the available information, replace the MC with a new, not reman unit, and you are done.

I agree, bubbles in the brake fluid would give repeatable response, not random. I confess I've tried replacing the master cylinder with rebuilds and have seen the same problem on two cylinders. If I can find a schematic diagram of the system I can see where the valves and other seals are and maybe guess where the problem is.

Since the problem happens only occasionally on slow, low pedal pressure I am guessing that one of the master cylinder piston seals (which are rubber cones and designed to seal better as pressure gets higher) is leaking or dirty or something.

Since you've suggested a new MC, any suggestions where one can be purchased, besides the dealer that is.

Thanks!
Mike

Did you bleed the new master when you installed it?

pull a working MC Booster combo from a late model rig, you'll get the dual diaphragm booster and the single chamber reservoir.

You can purchase new at any parts store, simply tell them you don't want a reman.

As suggested, you could upgrade to the 95-96 XJ MC/booster combo, or the WJ MC/booster conversion.