Discovery brake question.

[57jeeper]

I'm getting ready to start a new project, and 85 CJ7 Laredo that's seen better days
I'm not sure if this is the right section for this question though. My jeep has front disc brakes but no power booster, I scored a power booster with bracket from a member that west parting out an 85.
My question is, if I add power brakes where the stock brakes weren't power assist, do I need to change the proportion valve?
Thanks,

 

[Purple77CJ7]

No you shouldn't need to change out the propotioning valve. Adding a booster is assisting the brake application to make it easyer. It won't affect the brake bias, which is what the proportioning valve does.

 

[57jeeper]

Thanks Purple.

That's what I thought, but I had 3 people tell me that it was different for power assist, so the seed of doubt was planted.

 

[Torxhead]

Looking at a 4Wheel Drive Hardware catalog, it lists only one prop valve for the '82-'86 & '77-'78 and does not specify whether it is for power or manual, just stock brakes only.

 

[tarry99]

I'm getting ready to start a new project, and 85 CJ7 Laredo that's seen better days
I'm not sure if this is the right section for this question though. My jeep has front disc brakes but no power booster, I scored a power booster with bracket from a member that west parting out an 85.
My question is, if I add power brakes where the stock brakes weren't power assist, do I need to change the proportion valve?
Thanks,

The Proportioning valve no...............But here are some tips you need to look at.
The Brake Pedal ratio on a manual set of swing pedals versus a factory power setup will be different.
The ratio for a manual system is about 6.60:1 versus the Power system at about 4.71:1...........some pedals can be corrected,but some are bent so there is no room for correction. The difference is obvious and dangerous as using the higher ratio pedal makes the Power Boosted system very touchy to foot pressure............there are numerous people out there who have made this mistake and as a result hate there new power assisted brakes and don't even know why!

Also most Power assemblies usually require a 5 inch long linkage adapter that bolts directly through the firewall to the pedals with the booster and MC hanging off the front. The Booster and the adapter are two different bolt patterns.
This is where things get interesting as some Jeep adapters used the pedal ratio @ 1:1 while others off set the Booster & pedal rod and create another slower ratio which offsets its attachment point to the brake pedal.

Not impossible to correct just a lot of things to look at.

Last but not least the Master Cylinder Bore in a Power system is 1.125" versus its Manual Counterpart @ 1.00"........larger bore more volume and less pressure and the reverse for the smaller bore. Good Luck!

 

[57jeeper]

Thanks for the wealth of information. For now the non power brakes will suffice, and but good info for when I try it out.
I had no there were different pedal ratios,