Incorrect Master Cylinder ?

soft pedal? rear lockup? I'd assume the worst and go through everything, but I imagine you are already heading that way...

  So based on what you are saying with the different sized wheel cylinders, it would make sense that the front brakes are connected to the rear port of the M/C.

  You may be able to have your original master cylinder rebuilt, try white post restorations or John Plaskian (spelling?) in florida.

  I'm sure that you bench bled the M/C, and the lines, and have the adjusters setup correctly.  If the M/C that you installed is a 1" straight bore, then you may be getting too much pressure to the rear brakes because the system was designed for the smaller bore.  If you can't rebuild your original M/C, then you may need to install an adjustable proportioning valve in the rear brake line to manage the pressure and prevent the lockup.

Mike

Mike

The front 1" bore is very common and chances are that bendix is 1". That and the front wheel cyls determine "most" braking effort and ability. The 7/8" rear master would reduce volume and force to the rear for a given pedal-press; making the rear master bore 1" would increase rear braking, and yeah, lock up. 

But if the pedal is all mushy things are in bad shape...

If your'e going for 100% correct you'll want the original master cylinder, but if you wanna drive it you can make the 1" symmetrical master work by putting in smaller diameter rear wheel cylinders. Try next-smallest, 7/8" (14/16"). This is easier than messing with proportioning valve add-ons, which are not cheap and require plumbing anyway. Wheel cylinders are fairly cheap.

You can calculate relative pressures fairly easily but if you're not familiar with the math, call up some brake expert shops, I use C H Topping in Long Beach but there are others, and explain what you want. They can do that math easily.

I take it from your comments to recently got this (unknown) car? If so it is probably worth pulling off each wheel and drum and just looking around. Check for leaks (peel back the boot on each side of each wheel cyl), see if drums are OK, shoes not down to metal, etc. Wire brush it all clean. THe knowledge is cheap, and cheap insurance. You might end up making a shopping list.

I don't know how to ID a master cylinder even off the car and on the bench. Maybe there's a part number cast in? I doubt it's easy to find on the car. Very many physically interchange as far as mounting to the firewall/booster (is this manual brakes?), but the variables are:

* port configuration

* pushrod length

"Most" AMC master cyls are 1".

Port configuration, not just front and rear, but the threaded hole size varies. THats usually more trouble than the front/rear thing. I'm a bad person to ask about keeping brake systems stock, mine never are. Example: last time I did this, 1963 AMerican master cylinders were scarce, and $200 for a rebuilt: for 1964, $25 NEW! Guess which I chose. That catch: ports were the wrong size. But the lines were all rotten anyway, so I re-plumbed with standard 3/16" brake line and bought adapters. 

SO except for stock replacement most of my suggestions will be hacker-ish, as that's how I approach cars...