Rear brake problem

[HotRod777]

Hoping someone might be able to offer some ideas. My problem appears to be opposite of what usually happens. The car is a 66 with a brake system that I did not install. The problem I am having is the rear bias is too light. On a jack in drive I can barely stop the rear wheels from just the drag of the convertor.
The details I know are:
-Wilwood front disc kit (unknown model)
-Newer manual duel master cylinder, correct style for disc/drum (big/little reservoir. bore size unknown).
-Standard manual brake plumbing where the front uses a simple Tee and the rear heads straight to the back.
-New 11" drums with new 15/16" cylinders, new shoes & hardware kit.
(the rear stuff was all replaced by me chasing the issue)
The rears bleed correctly with great flow and I am familiar with adjusting drum brakes, I've even ran them tight to see if it helps.
Nothing I do seems to get "more" to the rear.
Would 7/8" cylinders that Diff Doc sells help? My thinking is that with a smaller cylinder the pistons will move farther giving the same fluid input... am I right???
I really would like to avoid installing a proportioning valve on the front wheels... especially in a daily driver type car.

 

[Don Frelier]

If everything else is set up correctly. I think you may need to swap the lines on the Master cylinder.
The big chamber goes to front brakes, small to the rear.
The pedal is firm and at the top?

 

[WileE]

p.s.I. the force applied the a certain area... you apply 100 pounds of force to the area of a 7/8" diameter, what is the psi ? you apply 100 pounds of force to a 15/16" diameter, what is the psi ? ( 7/8" area = .6 sq in, 15/16 area = .69 sq in )

 

[HotRod777]

If everything else is set up correctly. I think you may need to swap the lines on the Master cylinder.
The big chamber goes to front brakes, small to the rear.
The pedal is firm and at the top?

p.s.I. the force applied the a certain area... you apply 100 pounds of force to the area of a 7/8" diameter, what is the psi ? you apply 100 pounds of force to a 15/16" diameter, what is the psi ? ( 7/8" area = .6 sq in, 15/16 area = .69 sq in )

It never crossed my mind that someone could have plumbed it backwards... I'll look tonight. Although the different size line nuts (I would think) would protect from that happening. Yes, the car drives and stops "ok"... you just have to push really hard. The reason I noticed the lack of rear is doing a brake stand burn-out. I can push as hard as I can (a 40-ish big guy) and the car acts like it has a line-loc. Even at slight throttle I couldn't drag against the converter with the brakes. That's when I tried it up on a jack.
So... bigger might be better. (And, this is confirmed by Diff Dr, he replied to an email I sent that the 7/8" is used to reduce the rear... not add)

Is anyone aware of a 1" cylinder (and part number) that fits?

 

[Don Frelier]

I think regardless of the rear cylinder diameter you should easily be able to stop the rear wheels from spinning jacked up at idle in drive.
Sounds like you have no rear brakes at all, something is definitely wrong.

 

[Ron H]

Your setup does have a prop valve? Being disk/drum, it should. Also check the condition of the rear rubber hose (not collapsing) and the T-block on the axle (not clogged). You have good flow; but under pressure, could be a gremlin.

 

[MoparLeo]

Just wondering why, especially on a conversion you are resistant to an adjustable proportioning valve. Adjusting the front to rear bias is what it does... And your questions on a brake system would be better answered by the people, like Dr. Diff who assemble and sell brake systems if you are having issues. Make sense ?
11" drums are pretty big for a disc/drum set-up. Normally 10"
11"drums normally on 4 wheel HD drum set-ups.

 

[Don Frelier]

The adjustment is normally to reduce rear braking as the op originally stated.
The 11" drums are not the issue.

 

[khryslerkid]

Make sure there is some play in the brake pedal rod. If too long it might have the master cylinder piston out of place.

 

[Ron H]

Make sure there is some play in the brake pedal rod. If too long it might have the master cylinder piston out of place.

Yeah - was going to toss out rod travel into the MC (not shorted) and bore sizes vary btw a power and manual setup.