Hi All, I'm stumped on this one and could use some help. I swapped the stock kelsey hayes power disk brakes from a 1972 dart onto my 1963 valiant. Along with the swap I did a full rebuild of the front suspension with firm feel parts so I have the correct lower ball joints.

I used the proportioning valve from the 1972 dart and removed the old 4 wheel drum brake distribution block.

When I stand on the brakes I can push them basically to the end of their travel easily. I do have braking force but it is very weak, like not a chance I'll drive it weak.

The pedal feel is not any harder when the car is off as is usually the case with my other power brake mopars.

The master cylinder is a new raybestos MC36426 which shows as correct for the car with a 1_1/32nd bore.

The casting number on the bottom of the master appears to be 29893-C which I can't seem to cross reference to anything.

The rear is the stock 7_1/4 with 9" drums for now. The drum pads are low on material but the brakes bleed out fine and I replaced the wheel cylinders.

I've bled the bejeezus out of the system and have clear fluid at all four wheels. I kept going corner to corner bleeding until I had used 1.5x of the big bottles of brake fluid.

There are no visual leaks anywhere including in between the master cylinder and booster.

I'm using the stock 1963 brake pedal connected to the pushrod from the 1972 dart booster.

The pushrod between the booster and master is the original from the 1972 dart.

The bleeders on the calipers are pointing up.

Nothing makes any funny sounds.

The only things I can figure are I got a bad master cylinder that somehow is bad but doesn't leak, or I got a weird master cylinder in the wrong box with a tiny bore so I'm not pushing enough fluid.

Any and all input appreciated!

Help!!

-Jon
[img:left]https://photos.app.goo.gl/KcEjnDYw0Kr48uc12[/img]

did you adjust the master to booster rod air gap?.... .020-.060 airgap needed

Did you adjust the rear brakes? Too much clearance there and you will never get a good pedal. Easy test, set the E brake and see if the pedal firms up.

I would bet my last dollar that it is (as said) the rounded booster rod nub clearance. Use a valvetrain pushrod to measure the distance from the rear MC flat flange to the depth of the piston "thimble" then adj the booster rod protrusion out from the booster flat mounting flange to be .020" less than that. Still bad, then take off the MC lines & cap the ports with some brass plug fittings (be sure to get the right thread(s) or grab some junk fittings/lines & mash em shut close to the fittings. eng idling, with the MC capped the pedal should be rock hard with virtually no travel. holler back. EDIT this is with the MC bled out which I am assuming you have already done as a first (seperate) step in the bleeding process.

Thanks for the ideas so far.

The master cylinder is bled out.

I set the pushrod depth clearance but I'll re measure it when I get home today.

I'll try the idea with activating the parking brake as well and see if that changes anything.

I'll report back, but I'm open to any other ideas as well.

Thanks!
-Jon

I think he master cylinder bore is to big
I went through that years ago when I had to switch a master cylinder out at the races and I didn't know what size the old M.C. bore size was so I thought bigger bore was better, that car wouldn't stop worth beans after swapping to a 1 1/8 bore size off of a late model class B motor home chassis application
I found out later that I should have used a 15/16 or smaller for that application.
I weighed 225 at that time and worked for a telephone company climbing poles every day so my leg strength was good, I was almost bending the linkage to the master cylinder trying to get that old S/G car stop at Pomona
Are you using a power booster?
Good luck, let us know what you do to fix it, check the simple stuff first and then ,if you have to, change it to make it better

So I dug out the 72 Plymouth FSM (don't have my 72 Dodge one handy). Shows 1 1/32" MC bore for Valiant with discs. However, it only came with 10" rear drums and they are different than the 9" drums in more than just diameter.

You should be using a 1" or smaller bore master. Your stock pedal may have a different mount area than the 72 so the ratio may have changed a tad. I would agree that you may have a bad master! I have had to go through several to get one that worked right. The rebuilders are not doing the job right!