Power Brake Disc Conversion

[steve340]

Depending on how bad the rear brake locking is I would normally go down 1/8 to 3/16.
I even went down 1/4 on a light car.
Essentially you are making the back brakes weaker to the point they cannot over-power the traction of the tyres. Rear locking under braking in wet weather is going to try and turn the car around and is something to avoid completely.

The .906 is 1/32 drop and would be OK if your car is close to right. Your call if you want to try them.
It will be better but if you have been testing in the dry you would get a different result in the wet.

This is just a best guess. If you have been testing in the dry - go down at least 1/8.

 

[Dartman72]

Depending on how bad the rear brake locking is I would normally go down 1/8 to 3/16.
I even went down 1/4 on a light car.
Essentially you are making the back brakes weaker to the point they cannot over-power the traction of the tyres. Rear locking under braking in wet weather is going to try and turn the car around and is something to avoid completely.

The .906 is 1/32 drop and would be OK if your car is close to right. Your call if you want to try them.
It will be better but if you have been testing in the dry you would get a different result in the wet.

This is just a best guess. If you have been testing in the dry - go down at least 1/8.

I'm going to have someone sleeve 3/16 as I can always add pressure since my valve is all the way out.

 

[steve340]

Good call. It is nice to have the confidence knowing you can slam the brakes on and rely on those front discs to get the job done.
Once you get the cylinders done set the adjustable valve to the mid point and go up or down from there.
Basically you want to be at a point of no matter how hard you hit the brakes the rear does not cause a problem.

 

[steve340]

The 3/16 drop in wheel cylinder diameter should tone the back brakes down enough to eliminate any locking problems. Provided the rear brakes have correct spring placement, shoes etc.
You may find you are able to set the valve back to the maximum pressure going to the rears.
The smaller diameter wheel cylinders will not be able to apply as much force to the brake shoes at any given line pressure generated from the master cylinder.
I will go to a shopping mall car park or an open area on a wet day and do a few tests.
If they will not lock in the wet you won't have any problems in the dry.
When you get the job done and tested an update would be interesting.

 

[Dartman72]

I'll keep you posted. I'm reaching out to local shop to see if they can sleeve the cylinders.I'm not sure I'll get to it before spring as I'm in Minnesota and winter is coming - if we get snow before I get cylinders I'll have to wait until spring to test.

 

[Dartman72]

I'll keep you posted. I'm reaching out to local shop to see if they can sleeve the cylinders.I'm not sure I'll get to it before spring as I'm in Minnesota and winter is coming - if we get snow before I get cylinders I'll have to wait until spring to test.

 

[Dartman72]

Agh! I just recalled through the discussion of reducing pressure on rear brakes that when I talked to the guy that sold me this set up about where to install the manual proportioning valve he told me he preferred to bypass the stock proportioning valve and plumb direct from master Cyl through manual prop to rear brakes. I did that and now it just occurred to me that by doing so I eliminated the stock restriction. I have replumbed with the the stock and manual valve in series. I'm hoping to test this week and will let you all know!

 

[steve340]

Normally as mentioned you would only use one valve.
However give it a go it may work - or the back brakes could become to weak to work at all?
I am not sure what you are going to find.
Waiting for your update and feedback.

 

[Dartman72]

OK I'm all done ! Routing the rear brake line through the stock proportioning valve and then through the manual prop valve in series gave me the control I needed. I'm able to get rear brakes to lock up and then adjust down to no lock up which is all I needed. Things are adjusted and working well! Thanks to all for your ideas and interest!