ronw
Well-Known Member
Have a 68 Valiant that has original drum brakes and want to upgrade to original 68 Kelsey-Hayes set up. The question is do I need to change the prop valve and master cyc?
Kelsey-Hayes up grade
- Thread starter ronw
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scatpackbee
Well-Known Member
Yes, you will have to go with a disc brake master cylinder which has a larger reservoir at the rear for the front disc brakes and you will have to add a proportioning valve on the brake line to the rear drums.
The distribution block (with the low pressure switch) does not have to be changed.
The distribution block (with the low pressure switch) does not have to be changed.
ronw
Well-Known Member
So,
If I install a adjustable valve to the rear line that will work?
carfreak6970
Well-Known Member
If you go into the "How To Articles", there is a great write up on doing this conversion. There is also a couple of other hacks listed in the discussion of that article that will make the conversion a lot easier.
MOPAROFFICIAL
Oogliboogli
Looks nothing like that block.
scatpackbee
Well-Known Member
The lower picture is of the factory proportioning block that you need to install in the rear brake line.
An adjustable proportioning valve will work also in place of the factory one.
An adjustable proportioning valve will work also in place of the factory one.
carfreak6970
Well-Known Member
scatpackbee,
I dont believe this is a necessary. One of the things mentioned in the how too article for this conversation, slantsixdan mentioned you get the same effect of the proportioning block/adjustable proportioning valve if you use the smaller bore size cylinders in the rear. This saves you from finding/buying that proportioning valve and the trouble of modifying your brake lines for said valve.
ronw
Well-Known Member
Can you tell me what is the orginial size and what must be it reduced to
carfreak6970
Well-Known Member
Sorry, that is not really correct. The function of the proportioning valve is NOT to set front/rear brake balance, although everyone seems to assume that. The ACTUAL function of the PV is to retard the action of the rear drum brakes in a non-linear fashion so that they do not lock up faster than the front brakes. So, you're asking, what's the difference? The difference is that drum brakes due to their design are non-linear in their response to increased pressure on the brake pedal -- you press a little, they brake a little, but as you press more, they grab and brake much more aggressively (this is on purpose). Disc brakes, by their design, respond in a completely linear fashion -- you press more, they respond more, in a one-to-one ratio. When you have a mixed disc/drum system, you should be able to guess what happens -- you press a little, both the disc and the drum brake a little, but when you press harder, the drum brake responds faster, and (importantly) this is regardless of the front/rear braking proportion set by the size of the wheel cylinders or any other factor. So the so-called proportioning valve is really kind of a choke on the rear brake line -- the more you press the pedal, the less (proportionally) it sends to the rear brakes, to keep them from locking up prematurely (ideally, this would be determined using a smooth curve, but in actual Mopar practice it is really only a two-step deal). Using the smaller rear cylinder adjusts the overall front/rear balance, which you may or may not need, but used alone it costs you overall braking power, since you aren't getting the maximum results out of the brake system as a whole.
BillGrissom
Well-Known Member
"Retard" implies a time delay. That is the function of the "front metering valve" inside the "combination block". It delays actuation of the front disks to give the rear shoes time to contact the drum. But, you probably intend "retard" to mean moving along the x-axis in a "rear pressure" (y) vs "MC pressure" (x) plot (not time-related).
I'm not seeing the nonlinearity. The coefficient of friction between shoe and drum is constant, or at least as constant as it is for disks. There is an amplification effect from the leverage of the shoe pivots, but that is a fixed value (doesn't vary w/ pressure, i.e. linear). You can calculate it just from a statics model. Drum brakes thus give a stronger braking force for the same pressure input, but a fixed factor. That is why one needs to decrease pressure to the rears, and/or choose smaller diameter wheel cylinders. You might be thinking of the non-linearity between friction coefficient and braking factor. As many have found, if the coefficient goes up enough (ex. sticky shoes from leaking glycol fluid), the rears can lock-up from that "self-jamming" effect.
Agreed. I have seen a plot like that where the factory prop valve's "pressure out" vs "pressure in" is a 2-slope line.
If you do add an adjustable proportioning valve, you should install it downstream of the "pressure imbalance" switch (part of 1969 distribution block) or the switch won't work correctly. An adjustable valve is smart since the fixed setting of the factory valve is probably not correct for today's tires and after-market wheels.
clifftt
Well-Known Member
Bill, why would the factory valve be considered "probably not correct for today's tires" than yesteryear's? I'm not disagreeing, I just don't see why there would be any difference.
BillGrissom
Well-Known Member
You may be correct if the tires are the same size front to back, even if they do have much better grab today than the old bias-ply tires. But, there could be a difference in how the contact patch with the road changes under braking, as the weight shifts, since radial tires flex much differently than bias-ply. One thing for sure is that you should ignore old stopping distance specs since those were for much worse tires than we have today. Some people have wider tires on the rear and the factory prop valve ratio certainly wasn't for that. But, most people adding a prop valve are also changing brake components, so unlikely the factory ratio would be optimal. Indeed, it may not have been very optimal as these cars shipped. The ratio needed also changes with weight distribution. That is why some pickups and even minivans have a prop valve with an arm that senses the rear height so its ratio varies w/ the load. In sum, adjustable is always better if you can take the time to fine-tune the ratio (wet parking lot). If a cop asks what the hell you are doing, just tell him some dude on a website said it was OK to do skid tests.
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MVH is correct in saying the braking of a drum brake system is non-linear. The Mopar drum brakes are "self-energizing" meaning that one of the shoes tends to dig into the rotating drum. (Hence primary and secondary linings) If you've used a wood router then you've probably experienced this. When routing a board edge, moving in one direction (CW around a board exterior) produces better results than going CCW.
Drum brakes are more apt to lock up unexpectedly and give uneven braking from side-to-side.
I removed all factory valving on both my 68 cuda and 64 Valiant and ran the front MC line to a simple tee to split to the LF and RF brakes. The rear MC line goes to a Wildwood adjustable proportioning valve then on to the rear brakes. The factory valving was the same part whether it was a station wahon or a stripper /6 car.
No need to use smaller rear wheel cylinders (7/8) unless you are forced to AFTER driving the car.
I have tried 7/8, 1-1/32 and 1-1/8 on my 68 with 73-75 discs. For most people the 7/8 bore MC from a 73-75 power brake a-body run MANUALLY will give the best results. Unless you have a physical disability, there is no reason to run power brakes in my opinion. Just adds weight, takes up space, and introduces more things to go bad.
Drum brakes are more apt to lock up unexpectedly and give uneven braking from side-to-side.
I removed all factory valving on both my 68 cuda and 64 Valiant and ran the front MC line to a simple tee to split to the LF and RF brakes. The rear MC line goes to a Wildwood adjustable proportioning valve then on to the rear brakes. The factory valving was the same part whether it was a station wahon or a stripper /6 car.
No need to use smaller rear wheel cylinders (7/8) unless you are forced to AFTER driving the car.
I have tried 7/8, 1-1/32 and 1-1/8 on my 68 with 73-75 discs. For most people the 7/8 bore MC from a 73-75 power brake a-body run MANUALLY will give the best results. Unless you have a physical disability, there is no reason to run power brakes in my opinion. Just adds weight, takes up space, and introduces more things to go bad.
BillGrissom
Well-Known Member
There is an amplification effect w/ drum brakes, but that doesn't mean it is non-linear. Indeed, it should be the same amplification at all braking loads if the coefficient of friction (cof) is constant w/ load (is to a 1st approximation). The using a wood chisel on a lathe, or a router, the cof isn't constant. As the bit digs into the wood, it greatly increases, even jamming the machine. Drum brakes are more sensitive to variations in cof, which is why they can lock-up if the shoes get gummy (leaking brake fluid). But, most people are comparing defective drum brakes to new disk brakes, which isn't fair.
I did the same as Kosmic in my 64 & 65 Mopars, keeping the distribution block as a "front tee". I put a 1/4M-3/16F inv flare adapter in the top port and a 3/16 inv flare plug in the rear port. You can re-use your existing rear tubing w/ a 3/16 inv flare coupler. All adapters can be bought at Autozone.
I did the same as Kosmic in my 64 & 65 Mopars, keeping the distribution block as a "front tee". I put a 1/4M-3/16F inv flare adapter in the top port and a 3/16 inv flare plug in the rear port. You can re-use your existing rear tubing w/ a 3/16 inv flare coupler. All adapters can be bought at Autozone.
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