I have removed the ABS pump assembly from my 94 Grand Cherokee 5.2l as I don't want any ABS. However it seems I need to replace my master cylinder with a standard non ABS master cylinder as the rear wheels are now locking up before the front. It seems this problem is due to do a special valve system in the pistons.

Anyone know what standard master cylinder will fit onto my booster?

According to RockAuto, there is only 1 MC for the GC.
You most likely need a different proportioning valve or you plumbed and lines wrong.

The front port of the MC, you plumbed to the front or rear brakes?

The only thing I can add to this is my '67 Camaro (4 wheel drums) had a one way metering valve in the rear brake lines that delayed full pressure to the rears until you were on the brakes long enough for fluid to bleed thru. When I removed it, the rears locked up first.

I was trying to get more precise braking for gymkhana racing. I wound up modifying the metering port several times.

You need to swap the proportioning valve to one from a non-abs jeep. It's easy to do and if you're quick enough the fluid spill is minimal. I removed the abs from my XJ and ran into this same problem. Grabbed a non-abs prop valve from the junkyard and the front brake lines and very thing worked great once I had it swapped out and the lines run direct to each front wheel.

The port closest to the booster goes to the front brakes, this is how it was. I was thinking of trying it the other way around because the rears were also locking up a little bit even when the ABS was still connected.

Im also thinking of removing the valve and installing a manually adjustable valve on the rear line only.

You're fine to use the proportioning valve that came in it.
The only problem is you have to splice off of the hard lines after it because it isn't the same as the one pre ABS.
The rear brakes come out one port and the fronts come out of another. In the pre ABS proportioning valve, there are two front brake ports. So you'll have to Tee them off to go to both sides.
It's really not that tough.
You'll need a flaring tool and fittings.
Let me know if you need pics.

I have already done all that but the rears are locking up before the front if I immediately press the pedal, but if I press the pedal down slowly to 1st build a little pressure then press hard then no lock up. So I believe it is the Master Cylinder that is causing it and it is a brand new one.

The previous one did the same thing. according to the service manual there are center fed valves in the MC that allow the rear to take up some pressure first then the front. This is why I want to change the MC.

I have replaced the entire brake system... new rotors, new rear drums, new proportioning valve, new MC, etc..

I would like to swap the front and rear ports around on the MC just as an experiment and to see what happens. Anyone here done this?

OK ive tried everything, still the rears are locking up! with or without the ABS!

about to give up and disconnect the rear brakes!

I suggest you take it to some classic car/hotrod guys. They do a lot of front disc conversions on older drum systems and may know what your problem is.

I think you need a metering valve in the rear brake line.

I have had bad parts right out of the box. Very frustrating.

I tend to agree on that, thinking of installing an adjustable valve that is used in race cars. What I dont understand is WHY? I need to do such a mod!
I already have a restriction orifice adapter on the rear outlet port of the proportioning valve and cannot go any smaller on the restriction as the brakes will stop releasing.

Its driving me nuts. Two brake shops have looked at this and no one has been able to resolve it yet.

Yes I was wondering about the MC, bought it on line from Autoparts warehouse in NY... it could be faulty but it is not rebuildable due to the center fed valves!

No, a restricter is not what you need! It needs to be a one-way orifice valve so it releases fully when not being used. The one on my Camaro had a piston with a bypass port. When you hit the brakes, the piston shut off the fluid and directed it to the bypass port until pressure equalized, then it opened. On heavy braking, the car would slow down fast until the valve opened, then all 4 would lock up. Not good when you're braking for a high speed turn on the race course!

Know where I can order one from?

OK I am going to replace the Master cylinder with a used loan unit to see if the MC is the problem. If this does not solve the issue I will install the one-way valve

Try summitracing.com