First off this is my first post. I just found this forum and it is full of great information.

My problem...

1992 Bonneville SE with the upgraded 16" wheels and sportier suspension.

Brake pedal sinks to the floor where I have some braking if pushed hard on the pedal.
Jacked up the vehicle and had someone apply the brakes.
Right front has brakes meaning I can't turn the tire with the brakes applied.
Left front has no brakes meaning it turns freely.
Right rear has no brakes.
Left rear has brakes.

So what the heck is going wrong?

How'* the fluid level in the master cylinder?
Has the car been in storage?

Fluid level is fine. Car is driven daily.

Symptoms came on suddenly.

I'm reading about the hydraulics being a dual system. Is this front and back or is it RF LR and LF RR

As far as I know all GM cars with dual system is front/back. I'd start by bleeding the brakes at all four wheels.

Bleed the brakes a TON. Keep the MC full during the procedure. Start at the right rear, then left rear, then right front, then left front. Repeat several times.

OK...

I'm thinking about replacing the master cylinder. I hope it'* not something to do with the hydraulic unit seeing as it is $550+

I disconnected the negative battery terminal. Had a helper press the brake and then I opened up the left front bleeder screw. The right front has no braking pressure. Fluid came out very very slowly. We did this several times with no change. And I am still not getting any braking. The caliper piston doesn't squeeze seeing as there is no fluid pressure.

How do I bleed if there is no pressure? Shouldn't the fluid come streaming out?

Any more thoughts?

Yes, keep bleeding, you're not there yet.

If you can't get the lines bled, as above, replace the master cylinder, ensuring that you bench bleed it first.

I have no pressure in the two lines to bleed.

I open the screw and nothing comes out.

How can I bleed without pressure?

I've never seen a master cylinder so toasted it wouldn't pump fluid through an open line, so I believe you either have a petcock open or a broken brake line somewhere.

Ensure the master cylinder is full of fluid, pump the brakes for a minute or so, and then look under the car to see where the fluid you're pumping is going to.

An assistant will help this process a lot.

Agreed.

Look for any leaking fluid on the back tires from the wheel cylinders as well as all other areas of the car and if bleeding fails, it'* probably a master clyinder.