4 wheel disc on my '70 Challenger, I'm not sure of what part number the master Cylinder is or what bore size it is. And I'm not sure if the distribution block under the master Cylinder is correct for 4 wheel disc or if it's needed at all.
Pedal is mushy even after bleeding them, pedal travels 1/3 before feeling better, then requires alot of foot pressure to make the car stop. Any advice is appreciated, I'll check tonight for a part number on the master Cylinder.

first lets get/confirm the air is out of there then see if there is a parts mismatch. Starting with the MC take off the lines & plug the MC ports with a pair of brass fitting from the Edelman cabinet at your parts house (If you are a regular I am sure they would pull a MC & let you test fit fittings to get the right one(s) or call the MC mfr. & if the MC is good (& bled out) the pedal should be rock hard with virtually no travel. Some 3/8-24 (iirc) male plugs to plug ports further downstream in your diagnosis. EDIT Do they pump up? MORE EDIT calipers can be problematic to bleed out. what works for me is to have a helper stomp the pedal several times to agitate the fluid then with good timing I open the pedal as he starts his downstroke & some calipers need to be removed (still connected) & turned the right way to get the air out. its like the air breathing hole in the top of a tomb in the pyramids, the bleeder screw on the outside of the caliper dont have to be exactly at 12 0'clock but inside where the passage meets the bore must be at or close to 12 0'clock.

Not sure why that first photo posted sideways.

If I just pump the pedal several times it doesn't seem to get any more firm, if that's what you mean by "Do they pump up?"
This will be a long process as I don't have alot of time to work on it but I'll update this post when I can.

Is the distribution/proportion block needed with 4 wheel disc? If so, does mine look correct?

I had the same issues but with disc drum. I was running the wrong master cylinder. When I swapped the rear drums to discs (Wilwoods all round) it worked fine. No issues stopping almost 4000lb from 145. M/P used to offer two lightweight manual master cylinders I bought the one with the smallest bore. More pressure less volume and more pedal movement.
HTH's

I used a vacuum pump and it did take a while.

Just went through similar. What Robert said, then gravity bleed. Takes a while.

A mis-sized master cylinder will result in either a pedal that's tight with little travel and doesn't make enough pressure or one that's low but will pump up and provide plenty of pressure. A low mushy pedal that won't stop the car or pump up sounds like air. That doesn't mean it will work perfectly once the air is out.

Once the OEM brakes get changed out, the prop valves, metering blocks, combination valves and anything else in the system goes out the window, with the possible exception of a distribution block that does nothing but shut down half the system and turn on a warning light in the event of a line pressure failure. I might use an adjustable prop valve for fine tuning, but proper matching of components typically results in nothing extra being needed.

The master cylinder has to be plumbed correctly for front and rear depending on system requirements.

I've seen Wilwood calipers with small piston bores used in the wrong application and really foul up the balance and function of the system. They aren't all the same.

The master has to be bled first. Make up a few short pieces of brake line with the correct nuts (looks like a 1/2 and 9/16 on your unit, be careful with the thread pitch on the 9/16, 2 different ones used), turn the lines up and back into the master below the fluid level and slowly pump until the bubble are gone.

Don't pump the pedal until the majority of the air is out of the system. You'll make the fluid foamy and never get it bled until the air separates back out of the fluid. Gravity bleeding, vacuum, having a helper push the pedal down easy moving fluid through the system with the bleeder open, close the bleeder, let the pedal up, repeat, or a combination of those things will work.

Get one of these. About a $100 on ebay for the whole kit. Its the most borrowed tool I have. No more spongey pedal on any of my cars,no more wifey in the car pumping the brake pedal
https://www.ebay.com/itm/MOTIVE-PRO...-Brake-Bleeder-Kit-P-N-250/233130204348?

Or the auto parts store rent a tool

That prop valve should be open to insure you get the air out

Did you check the master cylinder to make sure it didn't or doesn't have a residual pressure valve in one of the ports
if that M.C was for a production car with 4 wheel disc brakes it won't, or shouldn't have one, if it was for a drum brake disc brake car it will have one for the drum side to keep around 10 lbs. residual pressure to prevent the brake shoes from forcing all the fluid back up the brake line into the M.C.

On my car when I went to 4 wheel disc brakes I had to change the MC. Went with a small bore MC that was under 1". I do not know the number - sorry.

Its a function of pedal ratio and MC bore size. See what you have a smaller bore will result in a firmer pedal that need more effort. Its all what you like. My racecar uses Will Wood all around. 1 1/32" piston. The pedal is "springy" but stops with easy effort. My Challenger has a 15/16" and the pedal is dead solid. But it takes way more foot effort.
Doug

What style of rotor, and what style of caliper and mount?
Is this a fresh install, or a system that has been on the car that has never been right?
The reason I ask, is some of the one piece hat/ rotors don’t take much heat at all to get “cupped”.
This will give you a squishy pedal until the rotor and caliper flex enough to get “square” pad contact. Any condition that causes the caliper and rotor not to be in perfect alignment will result in what you describe. High pedal effort is a pedal ratio or bore size issue. The two symptoms you describe tells me you have two separate issues.
The fact that pumping the pedal with the system closed doesn’t seem to affect pedal height, tells me you have some flexing going on at one or more of the caliper/ rotors, not an air issue.
Secure the rotors with the wheels off. Have someone watch the cal/ rotor while applying/ releasing the brake. If anything moves other than the brake pad, then you’ll have to get the issue sorted out.

Here's what I see:
Master Cylinder looks like a Chrysler. It will may be mounted to the firewall with an adapter plate (original 4 stud or bolt to 2 stud or bolt)
In the second photo, it looks like the brass distribution block where to lines from the master cylinder go. Chrysler calls it the Safety Switch, or a Combination Valve if it has additional functions.
The object with the Wilwood written on the knob looks like a Wilwood adjustable Proportioning Valve.
The small brass block in front of the proportioning valve could be another proportioning valve or a metering valve.

Lets go through the devices before touching on the issues:
The Master Cylinder's bore and stroke effects the amount of hydraulic fluid moved for a given amount of pedal travel.
How much movement and force at the calipers will depend on caliper piston size and number.
Therefore you need to figure out what calipers are on the front and rear to see if the master is appropriate.
If the master cylinder is appropriate, or you change to another one, if the masters were originally intended for a disk-drum application, there may be a low pressure check valve in the outlet for the rear. This can cause additional dragging on a disk as there's no springs pulling the pads back like a with drum brake. This is not your current problem so don't worry about it for now. Just something to keep in mind when adapting master cylinders to rear disk.

The Proportioning Valve does nothing until brake line pressure reaches relatively high pressure. Something around 300 psi is typical. Then it proportionally limits the increase of pressure to the rear brakes.
For example if the you begin to apply the brake for a stop light and pressure in the lines is at 250 psi, both the front and rear will be at 250 psi. As you press harder and the pressure from the master cylinder goes to 300 psi, all brakes still get 300 psi. But after that, the front will get full pressure, lets say 350 psi and the rear a percentage less, say 325 psi.

A proportioning valve is used to prevent rear lock up during hard braking as the weight shifts off the rear. Its very common with disk/drum combinations because drum brakes are self energizing. In other words they help themselves apply more pressure to the drum.

A Metering Valve is used on the front brakes of some vehicles to hold off the intial application of hydraulic pressure. Doing so makes sure the rear brakes start to act just a hair before the fronts, reducing the chance of front lock up on slick surfaces.

The Safety switch is just a little shuttle valve between the front and rear hydraulic circuits. When the pressure is unequal, the shuttle moves out of position and closes the switch. You see the light on the dash, and say oh Sh1t! which you probably already said and didn't need the light to tell you that.

Starting in 1970, Chrysler combined some of the valves into the Safety switch distribution block.
Look through this booklet for pictures and explantions. http://www.imperialclub.org/Repair/Lit/Master/274/page08.htm

Soft pedal could be air, a bad hose, something about the stroke. It would not be related to the proportioning valve function.
It could be related to using a master cylinder that doesn't move much fluid relative to the size of all the caliper pistons if the pistons have pulled back.

Extra braking force depends on what you are comparing with. Drum brakes require less pedal force to get the same stopping power (compared to disks of the approximately same size).
Friction material and surface of the disks also factor into the bite. Sometimes changing linings can make a noticible difference.
Diameter of the disk can make a difference. A caliper located further from the center will have more effect than the same one on smaller disk.

So again idea to see what disks and calipers are on the car. Would't hurt to pull the pads and see what they are too. There should be codes stamped into the edge of the lining or printed on the back of the metal plate.

Thanks guys, great info.

What do you think about that prop valve under my master cylinder, does it look correct for 4 wheel disc?
Is there any tricks to bleeding Wilwood calipers since they are 4 piston? I do the top 2 bleeds, then try to flip it up or over to do the bottom bleeds while they're facing up. I may not be doing it correctly.

Trendz, here's a couple pics. It's a fresh new build.

Mike, how do I ensure that it's open, just by pressing the pedal down?

I normally use a vacuum bleeder kit but I can have someone do the old pedal push/pump if that's what is needed.

Turn the valve all the way in and then out and split the difference to insure the Proportioning valve is opened.
On your deal with the after market rear disc brakes I would remove the rear calipers so I could hold them up as far as possible towards the sky to make sure all the air in the hoses and calipers where being forced into the top part of the calipers so it will be bled out when you bleed it the next time
I had to use this method on my airplane to get all the air out of those calipers
Air compresses fluids don't