[graygator]

What car?
My first guess is not to suspect the rear brakes at all.
Don't take it to someone else. Just take a step back and start over.

[Hotrodjohn71]

It's a 67 Firebird, all stock. 4 wheel power drum brakes.

[Sirrotica]

Mismatching brake linings can easily make drums brakes grab. Look at the edge of the brake lining, and see what designation the front and rear shoes have, FE, and FF are common designations. It tells what composition the lining is. Ideally you would like the front and the rear to match on a 4 wheel drum brake system.

Just to show you how much it matters, I used to drive 18 wheelers, the mechanics for some reason only replaced one half of a set of brakes on the steer axle. When braking it would literally rip the wheel out of your hands. They tried all kinds of adjustments, and parts swapping to cure the problem. Finally someone looked at the brake linings side to side, and determined that they didn't match. Replace the linings with new ones, and no pull at all. They screwed around with it nearly a week before they found the problem.

You say the problem showed up after changing brake shoes, so it's not far fetched to have a mismatch, all brake linings are not created equal, and it's not hard to check.

My preference is new linings, and new shoes that are riveted, not relined shoes, and not bonded linings. For over 50 years twisting wrenches my findings are that cheap linings, and relined bonded shoes will give you inferior brake jobs.

[Hotrodjohn71]

First. Thank you to all of you. I am carefully going over your advice

I've been driving the car around today. I've got a pedal that goes down about half way then the brakes come on quick and strong. Not grabbing after changing the rear shoes.

I would like to be able to fix that pedal drop and it will be great.

I can say with certainty that my master cylinder is a 1" bore. I measured the new one and the old one and both 1". All of the research I've done shows that the 4 wheel drum cars had a 1" bore, and also., that's what aftermarket lists.

Regarding the booster to Mc pushrod, I have just under 1/8" play measured by putting some body putty on the rod, assembling the setup., and measuring the crush height of the clay.

I have no fluid loss in the reservoirs, no tracable leaks, all adjusters are set up for a light drag on the wheels. All shoes are also new and of same brand.

I took a video of my Mc with the lid off :

https://youtube.com/shorts/5uTBwojVVnw?feature=share

and right when I press the pedal, there is a 'slight' geyser effect right when the pedal is pushed, and then no effect on release of pedal.

Are the 10lb residual pressure valves (either in-master, or wildwood inline inserts) designed to keep the inner cups in the wheel cylinders against the wheel cylinder push rods?

I'm wondering if that possible variable in 4 wheels could be causing the pedal drop.

There is no "spongy" when the pedal drops halfway, but I am certain the Mc is being engaged.

I found a Mc on ebay that looks in very good shape. I am hoping it will have the rpv's in good shape because I would like the original look.

Any thoughts?

[Sport69]

Make sure you have all the air bled from the lines (farthest to the closest bleeding order).

Double check your booster vacuum line and connections.

Power booster plunger rod is original to the car, and was originally inserted into the bottom hole on your brake pedal?

All brakes adjusted with a slight drag?

Zero leakage at the wheel cylinders?