Which master cylinder bore size to use for power disc Kelsey/Hayes

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cudajim

cudajim

cudajim
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My 67 Barracuda has stock Kelsey Hayes power disc brakes were stolen from a 68 Valiant. I got the brakes, master, prop valve all at once. Only problem is that the fronts don't grab well/rears lock up and the pedal is not so firm. The master also leaks from the top due to rust so it's time to change it anyway. My question is, what size bore should I buy and how do I get it to stop properly?
 
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cudajim said:
My 67 Barracuda has stock Kelsey Hayes power disc brakes stolen were from a 68 Valiant. I got the brakes, master, prop valve all at once. Only problem is that the fronts don't grab well/rears lock up and the pedal is not so firm. The master also leaks from the top due to rust so it's time to change it anyway. My question is, what size bore should I buy and how do I get it to stop properly?
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Sounds like air in the system... And considering the fronts don't grab the air is probably in the front half of the system...

Rears grabbing can be a lot of things.... drums out of round, newer shoes with the lining in the wrong location, (very common these days) grease on the the brake linings.. Brake adjustment... Lots of things...

I believe the OE Master for that application is 1" & unless I've made 100% sure everything is working correctly and I still don't like the pedal feel I tend to default to the OE M/C size...
 
I think 1" in is correct, but 15/16" will also work, manual or power. Sounds like some pistons are stuck, were the calipers rebuilt?
 
Cudajim says he has power disks so I don't think the pedal will be too hard.

If you didn't also grab the proportioning valve from the donor car (it is separate from the junction block) then it is not surprising that the rear brakes lock up. You can get an adjustable proportioning valve from Summit Racing. You can insert it into the line leading to the rear axle without having to cut or flare brake lines if you buy a short pre-flared section from the parts store and bend it into a loop, so it goes Junction Block Rear Output > Loop > Proportioning Valve > Existing Rear Line. The proportioning valve will wind up roughly side by side with the junction block, where you can reach down over the fender and turn the knob, but it isn't readily visible. In my experience, you need to turn down the rear braking by about 70%.
 
cudajim said:
My 67 Barracuda has stock Kelsey Hayes power disc brakes stolen were from a 68 Valiant. I got the brakes, master, prop valve all at once. Only problem is that the fronts don't grab well/rears lock up and the pedal is not so firm. The master also leaks from the top due to rust so it's time to change it anyway. My question is, what size bore should I buy and how do I get it to stop properly?
Click to expand...
Yes, they were rebuilt and I double checked them. Not pulling, just not stopping.
 
1 inch is stock
i used 15/16 like it much better
autozone lifetime warranty 62 bucks
 
mvh said:
Cudajim says he has power disks so I don't think the pedal will be too hard.

If you didn't also grab the proportioning valve from the donor car (it is separate from the junction block) then it is not surprising that the rear brakes lock up. You can get an adjustable proportioning valve from Summit Racing. You can insert it into the line leading to the rear axle without having to cut or flare brake lines if you buy a short pre-flared section from the parts store and bend it into a loop, so it goes Junction Block Rear Output > Loop > Proportioning Valve > Existing Rear Line. The proportioning valve will wind up roughly side by side with the junction block, where you can reach down over the fender and turn the knob, but it isn't readily visible. In my experience, you need to turn down the rear braking by about 70%.
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Yes, I got everything including the proportioning valve.
 
1 1/32 to 15/16 you will be fine.

1 1/32 is made in an aluminum housing with 4 bolt mounting

1 inch is stock for disk / drum and drum/drum and power or manual
 
i'd suspect the MC due the fact that it's leaking. who knows what else is going on up in there.

did it work fine at first and now it sucks, or has it never been optimal? was the MC from car and is it correct for a disc/drum system? (too many people put drum/drum MC's on and the bias isn't right)
 
Fwiw I use a 1.032 in. bore on manual brakes, stops perfectly.
 
Just to reiterate, I have power disc brakes, all factory setup for '67. It has never worked right, poor braking. Fronts won't lockup, rears will which makes it dangerous. I suspect either bad master, bad proportioning valve or bad bleeding of the system. In any case I want to change the master cylinder cause it's kind of crusty and looks bad.
 
Soooooooooo, the Val & Cuda are about the same weight....so you would find the m/c size that was used on the 68 Val.....& get one that size.
When discs are used with drums, a proportioning valve is need to reduce pressure to the drums.....because discs require about 50% more line pressure than drums...& you guessed it....the drums will lock up without some method of limiting pressure to them.
A prop valve is not needed with all-drums because the braking force can be regulated by lining area & wheel cyl diam.
 
cudajim said:
My 67 Barracuda has stock Kelsey Hayes power disc brakes were stolen from a 68 Valiant
Click to expand...

The 67 Dart, Valiant, and Barracuda all used the same brake parts.

Do you know if the brakes on the donor car worked properly?

Are you using 10x1.75 rear drums?

Did you get the following parts with the calipers and spindles

Disk drum master
Distribution block with the safety switch
Proportioning valve

cudajim said:
. Only problem is that the fronts don't grab well/rears lock up and the pedal is not so firm
Click to expand...

Did you replace the flex lines?
Did you rebuild the calipers?
If you bleed the brakes does fluid flow freely from all ports?
With a helper pressing the brake pedal down can you turn any of the wheels.

Not firm pedal can be from improper bleeding, master still has air in it?

Also power brakes typically feel mushy vs manual brakes. But obviously they should stop well with less pressure.



cudajim said:
The master also leaks from the top due to rust so it's time to change it anyway
Click to expand...

I assume your talking about the top edge where the seal sits.

cudajim said:
. My question is, what size bore should I buy and how do I get it to stop properly?
Click to expand...

The stock bore will stop correctly. Unfortunately 1" disk drum masters are MIA.

So you will have to use a 15/16" or 1- 1/32"

Either will work fine.


Something you need to do is verify that all 4 pistons per caliper are free to move.

That the rear wheel cylinders are the correct diameter.
 
I had to put 7/8 rear wheel cylinders on nine to get rid of rear wheel lock up. The original spec rear wheel cylinders were I think 15/16 which was a screw up when the cars were built.

I would get 15/16 master cylinder.

I pressure bleed mine to get all the air out.
 
if the system never worked right, you'd probably be better off systematically checking and eliminating each component from the list of offenders. dana has a great checklist there in the previous post.

the MC is a good start, and maybe that fixes it from the jump. but it would behoove you to take a deep dive into the system in the case of that just being band-aid for other underlying issues.

at least with power the size of the MC bore is a secondary concern.
 
340sFastback said:
I had to put 7/8 rear wheel cylinders on nine to get rid of rear wheel lock up. The original spec rear wheel cylinders were I think 15/16 which was a screw up when the cars were built.

I would get 15/16 master cylinder.

I pressure bleed mine to get all the air out.
Click to expand...
I used wheel cylinders for a 70,
1/2 ton pick up on a Duster yrs ago
Worked perfectly, no more lock up
 
mvh said:
Cudajim says he has power disks so I don't think the pedal will be too hard.
Click to expand...
Opps. Missed that. What I meant was firmness of pedal but all work. I've used 1974 Dart masters as well with good results.
 
340sFastback said:
I had to put 7/8 rear wheel cylinders on nine to get rid of rear wheel lock up. The original spec rear wheel cylinders were I think 15/16 which was a screw up when the cars were built.

I would get 15/16 master cylinder.

I pressure bleed mine to get all the air out.
Click to expand...
I am running 7/8 rear cylinders, it helped a little but they still lock up.
 
cudajim said:
I am running 7/8 rear cylinders, it helped a little but they still lock up.
Click to expand...
Make sure rear brakes in excellent shape, make sure master cylinder and calipers in excellent shape, and pressure bleed. I run EBC red pads in front and steel braid brake hoses too.
 
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