Back brakes will not work? on 1994 Cadillac DeVille

I just changed all 4 brake pads. But the back brakes will not work. The back brakes will not get any fluid.

Your car has drum brakes rear and disc brakes front. For safety the system has a front rear brake split in case of brake failure in the front or rear brake system The car splits the brakes in the case of hydraulic failure using a Pressure Differential Pressure Warning Switch and Valve.
Have you no fluid at either rear wheel? I wonder if the proportioning valve is seized and not allowing fluid to the rear. Locate the proportioning valve get someone to press the brake pedal, crack open the brake line going in to the proportioning valve and see if you have pressure, if not the problem is forward of the proportioning valve. If you have pressure going into the proportioning valve crack open each line coming out of it. With all four wheels at ride height with the brake pressure applied to the brake pedal each line should bleed out pressure when you crack open the line if not the proportioning valve is bad. If you have pressure coming out of the proportioning valve ensure the wheel cylinders are not seized but you said you don't have fluid getting back there.
I just went back again and checked my information thinking I looked up the wrong car it has information and pictures of drum brakes at the rear, obviously it is wrong.
There must be a proportioning valve that reduces brake pressure to the rear under severe braking. The proportioning valve may not be allowing fluid pressure to the rear wheels. Would love to hear what you find is the problem.
I wish I knew more about Cadillac. I will forward this to a friend and see if he can help. I went back again and looked at my information on your Cadillac. I see two types of brake master cylinder one has the proportioning valve as an integral part of the master cylinder so if this was functioning incorrectly you would in all likely hood have taken care of this when you replaced the master cylinder. A second master cylinder shows a different style master cylinder that does not incorporate an integral proportioning valve as part of the master cylinder. In either case if you ensure the resiviour is full and get a friend to press down on the brake pedal, if you crack open each brake line at the master cylinder do you get brake fluid expelled out each line when you bleed the master cylinder?
Cadillac brake bleeding procedure is Right Rear, Left Front, Left Rear, Right Front.
I will forward this now and hopefully Jim who is brilliant with GM products will be able to help.

Thanks Dan,
I will look for a telephone number I have at work and see if I can locate better vehicle specific information on this car. I have had to bleed other car's brake systems where a scan tool was required.
I was asking friends if they would know a problem with your cars' brakes that would cause no fluid to the rear, everyone said the same as we have discussed, ie proportioning valve issue, master cylinder proper brake bleeding procedure ect...
I wonder if Autoline (a service to professional garages) would be able to help. They answer vehicle specific questions (VIN number is needed) using a panel of auto technicians, factory training material, pattern failure database. Really good service inexpensive when we run into problems that are difficult to diagnose. Again I don't know if they will answer questions to non auto technicians but worth a call if all else fails
Actually autotechpat I have all 4 disc brakes, theres is no drum brakes on the car. There is no fluid coming out when I try to bleed the back brakes. I already bleed the front ones and they work just fine.
When I took off the lines I found out the Master Cylinder is not working. I ordered it from a parts store and its coming in Tuesday. I will let you know if that solves the problem with the brakes.
I replaced the master cylinder and the back brakes are atill not working its not geting any kind of fluid. Could it be the ABS? or the valves?
1 more answer , 4 more comments
See if your Caddy has ABS. If it does, then you need a scanner to bleed the brakes. I have run into this problem and when ever I deal with GM vehicle with ABS I have to use a scanner to bleed the brakes or I never get any pressure. i have seen other folks replace Master Cylinders etc until they realize that they need a scanner any time they work on the brakes on a GM car made after 1990. Especially if you ever leave the key on while bleeding or working on the brakes. This drives the ABS system crazy.

I hope that this helps. Good Luck. By the way, the is an ABS capable scanner not just a little one from a parts store.

Dear CadillacLover,

Do you have ABS in your Caddy? we need to firmly establish this first. The typer of scanner I am taking about costs $3000-4000, minimum. It is really for shops. But before we worry about that, lets make sure that you have ABS brakes. You can tell if you turn on your key with out starting the engine and there is usually an ABS in yellow letters next to your battery light etc. Good Luck!

Ok, since you do have ABS, you need an ABS capable scanner to correctly bleed these brakes. There is actually a brake bleeding menu on the scanner that opens up the ABS valve body. Also you need to make sure that your rear caliper parking brake functions are correctly adjusted before you attempt to bleed the brakes. If the brakes still won't pressure up with a scanner, the ABS valve body maybe really dirty so the valves won't seat correctly. In some cases this valve body has to be replaced, but let's stay positive. Good Luck
autotechpat I would really apperetiate it if you can ask your friend about the brake problem, Thank you.

dandd I didnt hear about a scanner before to bleed brakes. How big is the scanner? Could you please give me a picture or where I can buy the item?

Thank you to both for the help.
Yes I do have ABS in the car.
Its ovbously your proportional valve. I have the same problem on my 78 DeVille and am ordering a new proportional valve off of ebay for $60.00. Your valve could be clogged or it just could be not working right.
Any car that has ABS does not have a portioning valve, that is what ABS does, it is a much more sophisticated version of a proportioning valve plus much more. All a proportioning valve does is help to stop only the REAR brakes from locking up during a panic stop, an ABS Brake System controls all 4 wheels from locking up and on an individual, wheel by wheel basis.
When ever you bleed the brakes on any vehicle with ABS, you often have to re-set the ABS System. I just did it yesterday on a GM Silverado pickup, after I flushed out the Brake System. Re-setting the ABS on GM vehicles after brake work is quite common.