timmah22

My brakes are set up like a 67 and a have a 66. They work perfect, but I am curious....IF i find a decent deal on a 66 power master cylinder (unlikely), will I be able to use it with my 67 set up? Will it work with the proportioning valve? The distance between the brake lines is about 3 3/4". I don't know the break line size of the 67 set up. Will I have to buy new 1/4" lines? My booster is the original.

TCracingCA

It is a brake bias thing, but mistaken time after time as a proportioning valve!

It would work with what you have with a proper proportioning valve (3/16 or 1/4)

65GGvert

What's the difference in brake bias and proportioning?

TCracingCA

Bias would be sensed by this switch if you lost pressure to either your fronts or backs. It has a wire attached that sends to a light! I honestly don't know how well the Corvette feature works!

Proportioning controls the pressure (flow to the backs)! You can dial that in to compensate for the varied line sizes or engagement can be refined front to rear and also in consideration of varying weights front to year, like when burning thru a tank of gas, or shift or removing weight from the car changing the front to rear ratios. Even the proportioning valve can assist with brake grip based on tire size reactions to the pavement!

JohnZ

That device between your brake pipes appears to be a brake "combination valve"; it combines distribution, proportioning, metering, and differential pressure sensing, and doesn't belong on your car; there were hundreds of different calibrations of those valves made for front disc/rear drum systems - they weren't used on 4-wheel disc brake systems until the very late 70's when Corvettes gained enough weight that the default (designed-in) brake bias was no longer adequate.

Brake bias defines the difference in braking force between the front and rear systems; with Corvette 4-wheel disc brakes, larger caliper pistons in the front than in the rear calipers sets the bias (by design) at about 65% front, 35% rear. This is fixed and non-adjustable. For applications that require adjustability for setup differences (racing), the J56 brake package added an adjustable Kelsey-Hayes proportioning valve in the rear system to increase or decrease rear braking (hydraulic) force.

To the OP's question - yes, the '65-'66 power master cylinder will work just fine with your setup, as long as you get rid of that Bubba combination valve.

65GGvert

I've seen some 65-66 repro for sale at $850 from several vendors and $1100 from another. I just did a rebuild on mine last fall and I think the kit with pistons was around $40 if you can find a rebuildable used one. There were two different brake line sizes, but I don't remember if it changed in the middle of 65 or middle of 66. Sorry, but if you check the vendors specs, they note the differences. Of course with the modifications on yours, we don't know what size they used anyway.
I found this post from JohnZ from 09, hope it helps:

The '65 master used 3/16" lines/fittings, and the '66 casting used 1/4" lines/fittings. The GM service replacements came with a pair of adapter fittings so they'd work with either line size.

TCracingCA

I wouldn't recommend the J-56 proportioning valve (Kelsey hayes unit) unless you really have a really good one. Plus they have become super rare and heading toward expensive. I have three of them and the one going back on the car is practically brand new (corvette unit), the other is in fair shape and probably would continue to work (and is a Shelby unit), and the last is a pitted mess that maybe an outfit like White Post or SSBC could rebuild if they sleeved it!

Any of the NASCAR supply guys would have good proportioning valves! I get alot of stuff from Coleman!