Brake bias?

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oldkoot

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1967 Dart , power booster(new), KH 4 piston front disc brakes with 9" rear drums, no rear proportioning or hold off valve. The problem is it feels like the rear drums aren't really doing anything when it comes to stopping-I know 9" drums are the best but you should feel some braking. I've adjusted the rear shoes so they were lightly touching the drums and backed them off a couple of notches which made no difference, There is about an inch of pedal travel before anything happens then it feels like the fronts are doing all the work.
The pushrod from the booster to m/c clearance is set correctly( used the gauge from the master power brake service kit to verify adjustment), Raybestos disc/drum master cylinder is new, all new lines,flex hoses, wheels cylinders and springs. Rear shoes and drum are not glazed. Line pressure to the front disc brakes is 1100 psi, rear drums are 600 psi.
What am I missing here or this is normal??
Don
 
my car has the opisite problem back brakes work to good fronts dont work as good tons of play in the pedal also
 
Lift the back of the car up with a floor jack and place the trans in gear to make sure the back brakes are stopping the back wheels. I have seen wheel cylinders go bad were they leak air back in will not work--you bleed them they work for a while then stop working but do not leak brake fluid.

Just make sure both back wheels are high enough from the ground and always be sure to shut off the motor if the car starts to move or if the back brakes are not working shut the motor off, don't place the trans in to park with the back wheels spinning.

If the back brakes are working, not sure what more can be done.
 
Dodge Freak,
I've had it on the hoist and tried exactly what you mentioned, rears are working but it does require a fair bit of pedal. Rear wheel cylinders are new, reservoir level doesn't drop and the pedal stays firm. I bleed the rears a couple of times thinking it was air. Maybe it's just the nature of the beast?
 
I would suggest adding a perportion valve. You need the rear brakes to lock first in a panic stop situation.
 
Back brakes should work pretty easy, its called a resistual check valve (not spelled right) keeps 10 lbs of pressure on the back brake lines so the brakes work quicker.

Contact Summit racing-Jegs, a brake shop (but many might not have even been born when drum brakes were around). Goes in the line coming out of the master cylinder-sometimes built in to the M/C
 
I am having the same problem. I am using the factory distribution block/prop valve, whatever you want to call it. When I try to bleed the brakes, nothing gets through on the rear side of the block. The little piston/spring thing is not actuating. The way I understand it, the drums need to be preloaded and take more pressure to keep up. The little piston, spring thing in the prop valve is what makes this happen. I am thinking of gutting it (the rear part) and putting an aftermarket check valve in-line on the rear, which dodge freak suggested above.
 
I'm going to take exception with Redfish's contention that the rears need to lock first in a panic stop. Here are the reasons:

1. When stopping, weight is being transferred from the rear to the front (exact opposite for what ¼ milers want). A brake that is already biased towards lockup will be more likely to do so in a panic situation due to the speed with which significantly higher pedal, and resulting, hydraulic pressures.

2. When the front wheels lock, the car will tend to slide in the direction it was going at the time of lock up. When the rears lock first, the rear of the car tends to step out creating a broad slide. This dynamic is due to front weight bias in the A-body.

3. A locked wheel is not directionally stable and no longer able to respond to driver input. The natural response of most drivers (not necessarily the skilled ones here) to understeer are the correct ones. This is not so in the case of a broadslide or oversteer.

4. Put the brake closest to the weight. Most of the weight is on the front, but most of the braking effort on the front.

The most effective brake is not a locked one where the tire is being dragged like an eraser down the road. It is one that is just on the verge of locking up. This means that the brake and tire are operating at peak efficiency. Just like hard cornering will make the tires howl without leaving a black stripe, so too does maximum effort braking.

FWIW, I ride a motorcycle. When riding aggressively, I use the rear brake initially to set the bike up, then cram on front brake to whoa the critter down. If I lock up either wheel, I'm going to have a quick meeting with the ground. Even though most of the static weight of the bike is on the rear wheel, most of the braking effort is done by the front.
 
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