Front/rear balance after front disc swap

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rich006

rich006

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'74 Swinger, originally 4-wheel drum. I'm swapping to 8.25" rear (10"x2.5" drums) and factory '73-'76 front disc (11" rotor, 1976 calipers with the 2.75" pistons), manual brakes. For balanced braking, should I install the factory proportioning valve or use smaller rear wheel cylinders than the factory did, or both the proportioning valve AND the smaller wheel cylinders?

This is a later A-body and therefore on the heavy side. The vision for this car is a nice daily driver, not high performance. Wheels will probably be 15x7 with 215/60R15 tires if that matters. Factory setup according to 1974 FSM was 15/16" wheel cylinders with 1-1/32" master.

I know there's been a lot written about this already. I'm especially interested in hearing the experience of anyone with a '73-'76 car.
 
You should install a proportioning valve. Using the factory disk brake PV/safety switch is an easy swap. Or you can add an aftermarket PV in the rear brake line, but that's a lot more work. Changing the rear wheel cylinders is NOT the same thing. Yes, it affects the front/rear brake bias, but the valve does it dynamically. 10" rear drums with the factory front disks should work fine with the factory PV (that's the factory set-up) -- no need to swap wheel cylinders.
 
I installed an adjustable proportioning valve in the rear line under the hood at the master to balance my new system but if things get changed later (brakes, tire size, etc.) it's just a twist of the knob to reset to the new changes.
 
With bigger rear tires, 245 fronts and 275 rears, I found 7/8 w/c's and no Pvalve worked just fine. With 235s/295s I found 15/16 worked just fine, still with no Pvalve. But I have the KH 4-pistons up front.
I suppose installing an adjustable Pvalve would be the thing to do, and I confess I bought one. It's brand new and still in the bubble-pac 16 years later.
 
My son's '65 Cuda with the 73 and later front discs and 11x2.5" rear drums was brake torque heavy on the rear. (Sorry, I don't know the rear WC diameter.) The PO had installed an adjustable valve in the rear line but had it waaaaay too soft in the rear. We adjusted it to be better )More rear brake torque) and there was still some rear brake reduction set in the prop valve. So with your smaller 10" drums, you may not need much rear brake reduction; I would try your combo without the rear prop valve first; find some level gravel area and try braking with various pedal pressures; the gravel will make it very easy to tell which end is locking first.

We recently installed EBC YellowStuff pads up front with their higher coeffiicient of friction to further increase front brake torque, so as to get out more of the prop valve rear brake reduction adjusted out; lots of racing time (rally) makes me much prefer to get the F/R brake balance correct with CoF selection over using any prop valve. Using a lot of prop valve for rear brake torque reduction will result in the brake balance being right only in a narrow pedal pressure range; it will be out of balance F-to-R at higher and lower pedal pressures. (This is simply due to how the prop valves work....they tend to be a compromise adjustment.)

The higher CoF pads up front really make the braking action a dream on this car now. We have pretty much all of the rear prop valve adjusted out (waiting for good weather to do final checks); the F/R braking balance is much better over all pedal pressures.
 
rich006 said:
'74 Swinger, originally 4-wheel drum. I'm swapping to 8.25" rear (10"x2.5" drums) and factory '73-'76 front disc (11" rotor, 1976 calipers with the 2.75" pistons), manual brakes. For balanced braking, should I install the factory proportioning valve or use smaller rear wheel cylinders than the factory did, or both the proportioning valve AND the smaller wheel cylinders?
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Click to expand...

I have almost the EXACT same setup in my 68 cuda. Except I have an 8-3/4 rear with shortened C-body axles. 73-75 A-body front discs and 10 X 2-1/2" rear drums.
I also run four 215-60 x 15 tires on Mopar 15 x 7 steelies.

The car had the front discs (LBP) and the 10" x 1-3/4" rear drums (SBP) on it when I got it. I gathered parts and went to the current setup in the rear maybe 6-7 years ago. At that time, I just plumbed a Wilwood adj prop valve in the rear line and did NOT use any factory proportioning valve at all. I may have over compensated but I also installed 7/8" diameter rear wheel cylinders. In retrospect, I probably shouldn't have bothered with the smaller wheel cylinders but I think the adj valve was a last minute decision.

I run the brakes WITHOUT power assist and have had 7/8", 1-1/8" and now the current 1-1/32 aluminum MC in this car. I will say that the 7/8" gives the best overall feel and the 1-1/8" MC was too hard for me. I went with the 1-1/32 bore because I got a good deal on an old Direct Connection one at a swap meet. (it was anodized blue) It's a little firm but no where as tough as the big bore. The 7/8" bore is the one specified for 73-75 A-body power discs but run it manually!

I guess if I was you...I would ditch the factory prop valve, stay with the 15/16" rear cylinders, get an adjustable prop valve and install it on the LF front rail.
My philosophy is SIMPLE IS BETTER. Less connections=less chance of leaks. Summit for one, has the Wilwood for just over $40. You can rent a brake line flaring tool for free at most of the big parts store chains. I just did this on my 64 Val. (see pic)

You will lose the brake warning light that comes on when there is a difference in pressure (leak) between the front and rear circuits but your foot on the pedal will tell you when you have a leak. (ask me how I know) I try to "test" them every time I drive the car by pushing the brake pedal very firmly at a traffic light. If the pedal is NOT rock hard and gradually moves toward the floor you have a leak.

Your goal is to do some test stops and adjust until the fronts lock up just before the rears.
 

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1: Is there any change in a stock Drum/Drum Master Cylinder when switching to front disc/rear drum?
2: Can a guy get rid of the stock distribution block and just use the adjustable prop valve for the rear, and direct tie-in for the front discs with the stock master cylinder?

Thanks
 
If you are going to use the drum/drum MC, then you need to remove the residual valve in the front brake line port out of the MC.

Also, be aware that the drum-drum MC has a much smaller front reservoir fluid capacity than the ones made for the 73-76 disc/drum setups or for the Kelsey-Hayes disc system. So as the pads wear down, you will have to add fluid to the MC, and then remove the excess as you push the pistons back when you renew front pads. The correct disc/drum MC avoids this issue.

Yes, you can do as you suggest as long as you have the split for the front calipers.
 
'73-'75 A-bodies equipped with power disc brakes used a 15/16" (not 7/8") bore master cylinder.
 
Just bought a front power disc setup with a 1-1/8" bore master cylinder. My question is, do I have to, or should I change my existing brakes since I'm coming from a drum/drum setup?
 
I think you won't like the braking performance with a 1-1/8" MC.
I've got 2.75" calipers, 11.75" C-body rotors, Yellowstuff brake pads and a DrDiff 1-1/16" MC on my '73 Dart.
10" x 2.5" rear drums on a narrowed C-body axle.
While brake performance is good and very adequate for daily use, it still won't 'put you through the windshield' when braking so to speak.

I'm sure a 15/16" MC will provide enough hydraulic pressure to accomplish that.
 
This should read: Do I have to, or should I change my brake LINES when making the switch?
69wacuda said:
Just bought a front power disc setup with a 1-1/8" bore master cylinder. My question is, do I have to, or should I change my existing brakes since I'm coming from a drum/drum setup?
Click to expand...
 
No need the change the brake hard lines at all, except to plumb in any new parts. The diameters are the same.

You SHOULD for sure replace all 3 rubber lines when the get beyond 15-20 years old. The inner rubber liner breaks down where you can't see it and will eventually jam inside, either preventing brake action, or jamming one or more brake components in the ON position.
 
The only lines I replaced were along the rear axle housings, and that was only because my new 8.25" rear came from a salvage yard. By the way, I went with a reproduction stock proportioning valve and the braking action seems fine.
 
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