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Old 12-12-2008, 07:44 PM
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too little rear brakes

I have a 40 chevy coupe 11in GM disc up front 10in drum rear, wildwood prop valve on rear which is wide open. On a hard brake the fronts will lock, rears are working but not enough. Rear wheel cylinders were 7/8th I changed them to 15/16th some help. Rear is a chevy 10 bolt from a Nova. Idea's???
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Old 12-13-2008, 09:47 AM
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Quote:
Originally Posted by soups40
wildwood prop valve on rear which is wide open. Idea's???
Are you sure the proportioning valve is wide open?

They work just the opposite of what you would think.

Fully open is with the valve turned all the way CLOCKWISE. It should take about ten full turns to go from fully open to fully closed. Set it about half way and do a stop test. Adjust as required until the front brakes lock up just before the rear.

NOTE:

When the proportioning valve is fully closed (all the way counterclockwise) it will reduce the rear brake force slightly more than 50%.
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Old 12-13-2008, 03:01 PM
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soups40

Prop valve definitely open. I was always taught to set valve to lock up rears just before the fronts so you won't lose your steering ability. Unfortunately I can't get rear lock up at all. I believe it probably has something to do with tire size on rear 2.95x50x15 that's why I went with the larger wheel cylinders in the rear. Thanks for the response any other idea's
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Old 12-14-2008, 09:02 AM
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What are you using for a master cylinder? Manual or power brakes? Pedal assembly? Do you have a 10lb residual preassure valve in the rear line?

Having the rear brakes lock up first makes the car very likely to swap ends on you in a panic situation. The most stable braking arrangement is to have the fronts lock up slightly before the rears.

Andy
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Old 12-14-2008, 02:50 PM
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soups40

Using a 7in dual diaphragm power unit gm master with no residual valve. The master is firewall mounted well above brakes. Brakes apply on first pump with about 2in of pedal travel. Normal braking they feel pretty good, pedal pressure maybe slightly hi but when someone else drives the coupe there is no comment about the brakes. Once a hard stop is required pedal pressure is on the hi side just as the fronts lock up.
Engine running on a lift in gear will stall motor with even a slight application of brakes. This is why I wonder if the large rear tires are creating a need for more brake force than my 10in can provide.
Anyone have exp with braking with 2.95x50's in rear and 195x65 up front
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Old 12-14-2008, 07:47 PM
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Put a 10# check valve in the rear line, adjust up the brakes properly, and try it again. The brake setup you describe should be more than adequate if everything is installed properly. Just curious, how well does your parking brake hold the car?
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Old 12-15-2008, 01:44 PM
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What do you mean"engine running on a lift in gear will stall motor with even a slight application of brakes"???? How much vacuum do you have at idle and at cruise speed? It sounds like a vacuum leak or ???????
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Old 12-15-2008, 08:54 PM
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soups40

With the cam I have vacuum idle is on the low side. What I meant was that even with this vacuum the rear brakes are working enough to stall the engine.
On coast down with full vacuum the rear brakes won't lock up.
On inspection of the rears enough brake dust was present to indicate almost normal operation of the rears. The car stops pretty well pedal pressure a little high on hard stops.
My concern is why aren't the rears locking up with no proportioning to them?
My other coupe with almost the same set up needed almost 25% restriction to the rears. Tires were almost the same size all around on that one.
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