so we did the big front drum to disc conversion, which was one cluster of a time, and its still not quite right.

we used a 74 dart donor car for the 10" front disc set-up (indcluding proportioning valve) with power booster, additional 8-1/4 rear end with 11" drums off a d100?, and got all new lines, master cylinder, and rebuilt calipers. We bleed the thing, but the brakes were still a little bit spongy, but drivable.

And then we noticed that the rears locked up so we added a Russell adjustable proportioning valve and they got worse. he modified the stock proportioning valve so that it is only a connecter, we tried bleeding it some more, but it does the same thing. we tried adjusting the proportioning valve and it didn't seem to matter either way we adjusted it. I contacted edelbrock (who makes the adj. pro valve) and they weren't any help. Has anyone ever used this brand?

it feels stiff until you put it in gear and go anywhere, then it just goes to the floor. There are no leaks. When I try to stop it, there is a little bit of pressure at the beginning, but in order to come to a full stop, I need a lot of room and it goes all the way down. Pumping it only gives pressure for a moment. Someone suggested the master cylinder, but we tested that and it seems to be fine. The original MC went bad just before we did the brake swap and its not the same type of braking problem. Have we just not bled it enough?

We are running out of ideas...javascript:emoticon(':shock:')

~mrs. mctinker

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anything worth doing is worth re-doing, again and again and again...........

You bled the master cylinder, right? How about the rear brake shoe adjustment, is it good and tight? Sure those rear drums are right? Just some things I'd check.

D/W

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If it ain't broke, fix it!

Sounds like rear shoe adjustment to me.

The problem may lie in the master cylinder you are using not having the capacity to supply the wheel cylinders on those 11" rear brakes. I would try 10" drums on the rear....or a master cylinder of a d100 truck with 11" drums.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

I've changed 4 wheel 10" drums to wall-to-wall 11" drums several times with no MC change and no problems. I think the wheel cylinder difference is minimal... but you never know...

D/W

_________________

If it ain't broke, fix it!

I think you may be dealing with two separate problems here. One sounds like it's in the booster/master area (inconsistent stiff/sinking pedal), and one sounds like you need to rework the rear brakes.

I did a bunch of digging through piles of brake parts (new and used) with measuring tools, and digging through piles of parts and service literature (new and old) with a note pad when I was dialling in an A-body front disc changeover some years back and having only partial success with adjustable proportioning valves.

Here's what I learned:

Starting in '62, when the full-size cars first got the duo-servo brakes, most 10" and larger Mopar drum brakes used one wheel cylinder, with a bore of 15/16". The exception was police and taxi models, which got smaller-bore rear wheel cylinders, 13/16". The rationale behind smaller rear wheel cylinders with a heavy duty brake package was that it gave better brake system balance and reduced the tendency for early rear lockup under high pedal pressure. The fix worked great, and it still does. Disc brakes require higher line pressure than drum brakes, because they are not self-energizing. For that reason, with front discs, the rear drums "see" just about every stop as a high-pedal-pressure situation, and yeah, they like to lock up early. This was a common complaint in road tests of A-bodies with the newly-available disc brakes starting in mid '65 and continuing right through the '70s.

The particular disc brake changeover I was working with involved 4-piston discs up front and 10" drums in back, on a fairly heavy 111" wheelbase A-body. With all stock equipment, the rears locked very easily. As soon as I installed the 13/16" rear wheel cylinders, and with no other changes, the system started working beautifully. Very linear response to brake pedal pressure, and all four would lock up at virtually the same time and only if I really stood on the brake pedal.

I have NAPA United brake parts numbers in my notes. There are two part numbers for some of the bore sizes; if I remember correctly, one is "with inverted flare tube seat" and one is "without".

15/16" (stock exc. police) 37041 and 37235
7/8" 37236 and 20089
13/16" (police; I used these) 37696
3/4" 37863 only

11" drums are a lot of rear brake on an A-body, and since yours came off a truck, they may be even wider and have even bigger cylinder bores than the passenger car items. You may even want to change down to the 3/4" cylinders. But the 13/16" cylinders are known to have solved funky pedal behaviour and early/easy rear lockup in several changeovers I've done or followed along with as other people did them, in A- and B-bodies.

Another thing: Most of the A-body disc brake factory prop valves I've disassembled have been full of grit and gunk, and looked as though they'd been in that condition for years. Cleaning them out and installing new O-rings is possible and not too difficult but must be done carefully and thoughtfully. I don't think gutting it so that it's only a connector is a wise idea.

(FWIW, I don't run boosters on my A-body disc changeovers. Prefer 'em without.)