Front drum to disc conversion

[1370Pontiacman]

1971 lemans did a drum to disc conversion. Switch spindles with disc off of 70 lemans sport. New disc/drum mastercylinder. Hard brake pedal and hardly any brake. Do I need to change out proportion valve?

 

[melsg5]

Did you set the proper clearance between the master cylinder piston and the push rod?
You are positive you installed the correct master?
Proportion valve doesn't cause a hard pedal.
Do you have power brakes?

 

[cammerjeff]

I agree with melsg5, but also yes I believe you have to swap out the proportioning valve, 67-70 they used a proportioning valve for drum brakes, and if the car had disk brakes the7 added a small valve for the disk brakes on the outside of the master cylinder. Starting in 1971 they used a proportioning valve with that part built into it.

 

[melsg5]

cammerjeff is correct concerning the valve, see the attached page from the 1971 factory manual

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[avell007]

Sounds like a bad booster if you have power brakes. Jmo

 

[1370Pontiacman]

Well bleed brakes again no change ,but checked vacuum at back of carburetor, only pulling 7 lbs vacuum at 725 rpm.think I read somewhere it's supposed to be 18 lbs and no lower than 10lbs.any thoughts greatly appreciated

 

[1370Pontiacman]

1971 lemans did a power drum to disc conversion. Switch spindles with disc off of 70 lemans sport. New disc/drum mastercylinder. Hard brake pedal and hardly any brake. Do I need to change out proportion valve?

 

[1370Pontiacman]

Booster is holding vacuum

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[1370Pontiacman]

Did you set the proper clearance between the master cylinder piston and the push rod?
You are positive you installed the correct master?
Proportion valve doesn't cause a hard pedal.
Do you have power brakes?

They are power and booster is holding vacuum

 

[melsg5]

Well bleed brakes again no change ,but checked vacuum at back of carburetor, only pulling 7 lbs vacuum at 725 rpm.think I read somewhere it's supposed to be 18 lbs and no lower than 10lbs.any thoughts greatly appreciated

What is the vacuum reading at the manifold and at what rpm?
Did you read the post that you need the disc proportioning valve?

 

[1370Pontiacman]

What is the vacuum reading at the manifold and at what rpm?
Did you read the post that you need the disc proportioning valve?

I have the proportion valve on back order from opg.will update upon arrival